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/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);