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/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');