feat(ui): global message dialog to replace window.alert()

useMessageDialogStore + <MessageDialogHost /> (mounted once in App.tsx)
give a themed in-app dialog callable from anywhere — React components
and plain .ts modules alike via showMessageDialog(msg, {kind}). Swaps
the native alert() calls in FileExplorer (import errors) and the
desktop menu (.vlx open errors, updater status) for it; the pro overlay
can reuse the same store.

frontend/src/App.tsx

@@ -34,6 +34,7 @@ import { LocaleSync } from './i18n/LocaleSync';
 import { NON_DEFAULT_LOCALES } from './i18n/config';
 import { useProRoutes } from './lib/proRoutes';
 import { triggerSessionCheck } from './lib/proSession';
+import { MessageDialogHost } from './components/ui/MessageDialogHost';
 import './App.css';
 
 /**
@@ -164,6 +165,9 @@ function App() {
           <Route path="/en/*" element={<EnPrefixRedirect />} />
         </Routes>
       </LocaleSync>
+      {/* Global alert() replacement — opened from anywhere (React or plain
+          .ts) via showMessageDialog() in store/useMessageDialogStore. */}
+      <MessageDialogHost />
     </Router>
   );
 }

frontend/src/components/editor/FileExplorer.tsx

@@ -11,6 +11,7 @@ import {
 import type { BoardKind } from '../../types/board';
 import { boardDisplayName } from '../../types/board';
 import { importProjectFile, PROJECT_FILE_ACCEPT } from '../../utils/importProject';
+import { showMessageDialog } from '../../store/useMessageDialogStore';
 import './FileExplorer.css';
 
 // SVG icons — same style as EditorToolbar (stroke-based, 16x16)
@@ -284,7 +285,7 @@ export const FileExplorer: React.FC<FileExplorerProps> = ({ onSaveClick, onNewCl
         }
       }
     } catch (err) {
-      window.alert((err as Error).message);
+      showMessageDialog((err as Error).message, { kind: 'error' });
     }
   }, []);
 

frontend/src/components/ui/MessageDialogHost.tsx

@@ -0,0 +1,119 @@
+/**
+ * MessageDialogHost — renders the global message dialog driven by
+ * useMessageDialogStore (the replacement for window.alert()).
+ *
+ * Mounted once in App.tsx so it is available on every page, in web and
+ * desktop builds, and to the pro overlay. Styling follows the dark modal
+ * convention used by FlashModal / ShareModal.
+ */
+
+import { useEffect, useRef } from 'react';
+import { createPortal } from 'react-dom';
+import { useMessageDialogStore, type MessageDialogKind } from '../../store/useMessageDialogStore';
+
+const ACCENTS: Record<MessageDialogKind, { bg: string; fg: string; icon: string }> = {
+  info: { bg: '#16283a', fg: '#7cc4ff', icon: 'ℹ' },
+  success: { bg: '#143824', fg: '#7ee87e', icon: '✓' },
+  error: { bg: '#3a1a1a', fg: '#ff8585', icon: '⚠' },
+};
+
+export const MessageDialogHost = () => {
+  const { open, kind, title, message, close } = useMessageDialogStore();
+  const okRef = useRef<HTMLButtonElement | null>(null);
+
+  useEffect(() => {
+    if (!open) return;
+    // Focus OK so Enter dismisses, matching the native alert() flow.
+    okRef.current?.focus();
+    const onKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') close();
+    };
+    window.addEventListener('keydown', onKeyDown);
+    return () => window.removeEventListener('keydown', onKeyDown);
+  }, [open, close]);
+
+  if (!open) return null;
+
+  const accent = ACCENTS[kind];
+
+  return createPortal(
+    <div
+      role="dialog"
+      aria-modal="true"
+      onClick={close}
+      style={{
+        position: 'fixed',
+        inset: 0,
+        background: 'rgba(0, 0, 0, 0.6)',
+        zIndex: 9700,
+        display: 'flex',
+        alignItems: 'center',
+        justifyContent: 'center',
+      }}
+    >
+      <div
+        onClick={(e) => e.stopPropagation()}
+        style={{
+          width: 440,
+          maxWidth: 'calc(100vw - 32px)',
+          maxHeight: 'calc(100vh - 64px)',
+          background: '#1a1d24',
+          color: '#e6e6e9',
+          border: '1px solid #2c2c33',
+          borderRadius: 8,
+          padding: 20,
+          boxShadow: '0 12px 36px rgba(0,0,0,0.7)',
+          display: 'flex',
+          flexDirection: 'column',
+          gap: 14,
+          fontFamily: '-apple-system, BlinkMacSystemFont, sans-serif',
+        }}
+      >
+        {title && (
+          <h2 style={{ margin: 0, fontSize: 15, fontWeight: 600 }}>{title}</h2>
+        )}
+
+        <div
+          style={{
+            display: 'flex',
+            gap: 10,
+            alignItems: 'flex-start',
+            padding: 12,
+            background: accent.bg,
+            color: accent.fg,
+            borderRadius: 4,
+            fontSize: 13,
+            lineHeight: 1.5,
+          }}
+        >
+          <span aria-hidden style={{ fontSize: 15, lineHeight: '19px' }}>
+            {accent.icon}
+          </span>
+          <span style={{ whiteSpace: 'pre-wrap', overflowY: 'auto' }}>{message}</span>
+        </div>
+
+        <div style={{ display: 'flex', justifyContent: 'flex-end' }}>
+          <button
+            ref={okRef}
+            type="button"
+            onClick={close}
+            style={{
+              padding: '7px 20px',
+              fontSize: 13,
+              fontWeight: 600,
+              color: 'white',
+              background: 'linear-gradient(135deg, #007acc 0%, #005ea1 100%)',
+              border: '1px solid #005ea1',
+              borderRadius: 4,
+              cursor: 'pointer',
+              fontFamily: 'inherit',
+            }}
+          >
+            OK
+          </button>
+        </div>
+      </div>
+    </div>,
+    document.body,
+  );
+};

frontend/src/desktop/menu.ts

@@ -29,6 +29,7 @@ import { useSimulatorStore } from '../store/useSimulatorStore';
 import { useEditorStore } from '../store/useEditorStore';
 import { useProjectStore } from '../store/useProjectStore';
 import { useCompileLogsStore } from '../store/useCompileLogsStore';
+import { showMessageDialog } from '../store/useMessageDialogStore';
 import { switchLocale } from '../i18n/path';
 import { LOCALES, type Locale } from '../i18n/config';
 
@@ -148,8 +149,9 @@ function pickAndImportVlx(): void {
       try {
         await importVlxFile(file);
       } catch (err) {
-        // eslint-disable-next-line no-alert
-        alert(`Failed to open .vlx: ${(err as Error).message}`);
+        showMessageDialog(`Failed to open .vlx: ${(err as Error).message}`, {
+          kind: 'error',
+        });
       }
     }
     document.body.removeChild(input);
@@ -181,7 +183,6 @@ function newProject(): void {
     project.currentProject !== null;
 
   if (hasWork) {
-    // eslint-disable-next-line no-alert
     const ok = window.confirm(
       'Start a new project? Any unsaved changes will be lost.',
     );
@@ -230,19 +231,18 @@ async function checkForUpdates(): Promise<void> {
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const updater = (window as any).__TAURI__?.updater;
     if (!updater?.check) {
-      // eslint-disable-next-line no-alert
-      alert('Update plugin not available in this build.');
+      showMessageDialog('Update plugin not available in this build.');
       return;
     }
     const update = await updater.check();
     if (update) {
       await update.downloadAndInstall();
     } else {
-      // eslint-disable-next-line no-alert
-      alert('Velxio Desktop is up to date.');
+      showMessageDialog('Velxio Desktop is up to date.', { kind: 'success' });
     }
   } catch (err) {
-    // eslint-disable-next-line no-alert
-    alert(`Update check failed: ${(err as Error).message}`);
+    showMessageDialog(`Update check failed: ${(err as Error).message}`, {
+      kind: 'error',
+    });
   }
 }

frontend/src/store/useMessageDialogStore.ts

@@ -0,0 +1,50 @@
+/**
+ * Global message dialog store — the in-app replacement for window.alert().
+ *
+ * Any code can open the dialog:
+ *   - React components: `showMessageDialog('...', { kind: 'error' })`
+ *   - Plain .ts modules (desktop menu handlers, services): same call —
+ *     zustand stores work outside React via getState().
+ *
+ * The dialog itself is rendered by <MessageDialogHost />, mounted once in
+ * App.tsx. The pro overlay reuses this store via `@velxio/store/...`.
+ */
+
+import { create } from 'zustand';
+
+export type MessageDialogKind = 'info' | 'success' | 'error';
+
+export interface MessageDialogOptions {
+  kind?: MessageDialogKind;
+  /** Optional header line. Callers pass an already-translated string. */
+  title?: string;
+}
+
+interface MessageDialogState {
+  open: boolean;
+  kind: MessageDialogKind;
+  title: string | null;
+  message: string;
+  show: (message: string, opts?: MessageDialogOptions) => void;
+  close: () => void;
+}
+
+export const useMessageDialogStore = create<MessageDialogState>((set) => ({
+  open: false,
+  kind: 'info',
+  title: null,
+  message: '',
+  show: (message, opts) =>
+    set({
+      open: true,
+      message,
+      kind: opts?.kind ?? 'info',
+      title: opts?.title ?? null,
+    }),
+  close: () => set({ open: false }),
+}));
+
+/** Imperative helper so non-React callers don't need to know zustand. */
+export function showMessageDialog(message: string, opts?: MessageDialogOptions): void {
+  useMessageDialogStore.getState().show(message, opts);
+}