fix(ipc): wire preload bridge so menu Open file actually loads in editor + preview

Two related defects were breaking the 'open file' flow:

1. The BrowserWindow webPreferences had no preload path AND
   contextIsolation: false, so contextBridge.exposeInMainWorld
   threw on load and window.electronAPI was never set. Result:
   every renderer-side ipc.* call returned CHANNEL_MISSING and
   silently no-op'd. Toolbar Open/Save, the file menu, every
   dialog — all broken.

2. The 'file.opened' IPC bridge in register-menu-commands.ts
   dispatched to a 'file.opened' command that was never registered
   as a handler. Even if the preload had worked, the main menu's
   File -> Open would have been a no-op once it reached the
   command store.

Fixes:
- window/index.js: add preload path; set contextIsolation: true
  (required for contextBridge) and nodeIntegration: false
  (renderer no longer needs direct Node access; everything goes
  through the whitelisted preload bridge).
- file-store.ts: add openFileFromMain(path, content) that opens
  a tab + editor buffer from content the main process already read
  (skips the renderer's ipc.file.read since main has the bytes).
  Preserves existing buffer content if the file is already open.
- register-menu-commands.ts: register 'file.opened' to call
  openFileFromMain. Drop the dead 'command.palette' bridge
  (no consumer, no UI, no handler).
- tests: two new cases for openFileFromMain — verifies it does
  not call ipc.file.read and does not clobber user edits on
  re-open.

Verified end-to-end via the run-desktop driver: triggering
file-opened from the main process with a test .md now shows the
content in BOTH the CodeMirror editor and the rendered preview
pane (plus the Outline panel picks up the H1). 497 tests pass.

src/main/window/index.js

@@ -16,8 +16,15 @@ function createMainWindow() {
     y: bounds.y,
     show: true,
     webPreferences: {
-      nodeIntegration: true,
+      // The preload script exposes `window.electronAPI` — the only IPC
-      contextIsolation: false,
+      // bridge the renderer uses. Without this, every renderer call returns
+      // CHANNEL_MISSING and file/folder/save all silently no-op.
+      preload: path.join(__dirname, '../../preload.js'),
+      // contextIsolation MUST be true for the preload's contextBridge
+      // exposeInMainWorld call to succeed. Without it, the preload throws
+      // on load and the renderer never gets the IPC bridge.
+      contextIsolation: true,
+      nodeIntegration: false,
       spellcheck: true
     },
     icon: path.join(__dirname, '../../../assets/icon.png')

src/renderer/lib/commands/register-menu-commands.ts

@@ -102,6 +102,10 @@ export function registerMenuCommands(): void {
   });
   register('view.toggleSidebar', () => useAppStore.getState().toggleSidebar());
   register('view.togglePreview', () => useAppStore.getState().togglePreview());
+  register('file.opened', (payload?: { path?: string; content?: string }) => {
+    if (!payload?.path) return;
+    void useFileStore.getState().openFileFromMain(payload.path, payload.content ?? '');
+  });
 }
 
 export function useRegisterMenuCommands(): void {
@@ -118,7 +122,6 @@ export function useRegisterMenuCommands(): void {
 export function useBridgeNativeMenu(): void {
   useMenuAction('file-save', 'file.save');
   useMenuAction('toggle-preview', 'view.togglePreview');
-  useMenuAction('toggle-command-palette', 'command.palette');
   useMenuAction('toggle-sidebar-panel', 'view.sidebarPanel', (panel) => panel as string);
   useMenuAction('toggle-bottom-panel', 'view.bottomPanel');
   useMenuAction('toggle-find', 'find.toggle');

src/renderer/stores/file-store.ts

@@ -36,6 +36,7 @@ interface FileState {
   loadChildren: (dirPath: string) => Promise<void>;
   toggleExpanded: (dirPath: string) => void;
   openFile: (filePath: string) => Promise<void>;
+  openFileFromMain: (filePath: string, content: string) => Promise<void>;
   closeTab: (id: string) => void;
   setActiveTab: (id: string) => void;
   markTabClean: (id: string) => void;
@@ -165,6 +166,25 @@ export const useFileStore = create<FileState>()(
       });
     },
 
+    // Used when the main process has already read the file (e.g. file-opened
+    // IPC from File → Open menu). Skips the renderer-side IPC read since main
+    // already has the content. Preserves the editor's current buffer content
+    // if the file is already open.
+    openFileFromMain: async (filePath, content) => {
+      const existing = useFileStore.getState().openTabs.find((t) => t.id === filePath);
+      if (existing) {
+        set((s) => { s.activeTabId = filePath; });
+        return;
+      }
+
+      const title = filePath.split('/').pop() ?? filePath;
+      useEditorStore.getState().openBuffer(filePath, filePath, content);
+      set((s) => {
+        s.openTabs.push({ id: filePath, path: filePath, title, dirty: false });
+        s.activeTabId = filePath;
+      });
+    },
+
     closeTab: (id) => {
       set((s) => {
         const idx = s.openTabs.findIndex((t) => t.id === id);

tests/unit/stores/file-store.test.ts

@@ -232,6 +232,37 @@ describe('useFileStore', () => {
     expect(useEditorStore.getState().buffers.size).toBe(0);
   });
 
+  // --- openFileFromMain ---
+
+  it('openFileFromMain does NOT call ipc.file.read (main already sent content)', async () => {
+    await useFileStore.getState().openFileFromMain('/root/from-menu.md', '# from menu');
+
+    expect(fakeRead).not.toHaveBeenCalled();
+    const buf = useEditorStore.getState().buffers.get('/root/from-menu.md');
+    expect(buf).not.toBeUndefined();
+    expect(buf!.content).toBe('# from menu');
+    const tabs = useFileStore.getState().openTabs;
+    expect(tabs).toHaveLength(1);
+    expect(tabs[0].path).toBe('/root/from-menu.md');
+    expect(tabs[0].title).toBe('from-menu.md');
+    expect(useFileStore.getState().activeTabId).toBe('/root/from-menu.md');
+  });
+
+  it('openFileFromMain on an already-open file does not overwrite the editor buffer', async () => {
+    // Open once via the menu path
+    await useFileStore.getState().openFileFromMain('/root/x.md', '# menu version');
+    // User then types something in the editor (simulated via updateContent)
+    useEditorStore.getState().updateContent('/root/x.md', '# user edited');
+
+    // Menu sends the same file again (e.g. user picks it from Recent Files)
+    await useFileStore.getState().openFileFromMain('/root/x.md', '# menu version');
+
+    // Buffer keeps the user's edits — the menu re-open must not clobber them
+    expect(useEditorStore.getState().buffers.get('/root/x.md')!.content).toBe('# user edited');
+    expect(useFileStore.getState().openTabs).toHaveLength(1);
+    expect(useFileStore.getState().activeTabId).toBe('/root/x.md');
+  });
+
   // --- closeTab ---
 
   it('closeTab removes the tab and updates activeTabId to the previous neighbor', async () => {