feat(renderer): register menu commands in AppShell (Phase 6 wiring)

- useRegisterMenuCommands: registers file.open, file.save, file.closeTab,
  tab.next, tab.prev, view.toggleSidebar, view.togglePreview in command store
- useBridgeNativeMenu: useMenuAction for 13 native-menu channels
  (file-save, toggle-preview, load-template-menu, etc.)
- AppShell calls both on mount
- integration test mock extended to include ipc.menu.on
- 145/145 tests pass (was 105)

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

@@ -0,0 +1,74 @@
+import { useEffect } from 'react';
+import { useCommandStore } from '@/stores/command-store';
+import { useFileStore } from '@/stores/file-store';
+import { useAppStore } from '@/stores/app-store';
+import { useMenuAction } from '@/hooks/use-menu-action';
+
+/**
+ * Register all Phase 6 menu commands in the command store, and bridge
+ * the native menu IPC channels to the matching command ids.
+ *
+ * Phase 6 scope: file/view/tab commands with direct store mappings.
+ * Phase 7+ will add the dialog-driven commands (export, settings, etc.)
+ * once the corresponding modals exist.
+ */
+export function useRegisterMenuCommands(): void {
+  // Register handlers in the command store.
+  useEffect(() => {
+    const { register } = useCommandStore.getState();
+    register('file.open', () => {
+      void useFileStore.getState().openFileDialog();
+    });
+    register('file.openFolder', () => {
+      void useFileStore.getState().openFolderDialog();
+    });
+    register('file.save', () => {
+      void useFileStore.getState().saveActiveBuffer();
+    });
+    register('file.closeTab', () => {
+      const { activeTabId, closeTab } = useFileStore.getState();
+      if (activeTabId) closeTab(activeTabId);
+    });
+    register('tab.next', () => {
+      const { openTabs, activeTabId, setActiveTab } = useFileStore.getState();
+      if (openTabs.length === 0) return;
+      const idx = activeTabId ? openTabs.findIndex((t) => t.id === activeTabId) : 0;
+      const safeIdx = idx === -1 ? 0 : idx;
+      const nextIdx = safeIdx >= openTabs.length - 1 ? 0 : safeIdx + 1;
+      setActiveTab(openTabs[nextIdx].id);
+    });
+    register('tab.prev', () => {
+      const { openTabs, activeTabId, setActiveTab } = useFileStore.getState();
+      if (openTabs.length === 0) return;
+      const idx = activeTabId ? openTabs.findIndex((t) => t.id === activeTabId) : 0;
+      const safeIdx = idx === -1 ? 0 : idx;
+      const nextIdx = safeIdx <= 0 ? openTabs.length - 1 : safeIdx - 1;
+      setActiveTab(openTabs[nextIdx].id);
+    });
+    register('view.toggleSidebar', () => useAppStore.getState().toggleSidebar());
+    register('view.togglePreview', () => useAppStore.getState().togglePreview());
+  }, []);
+}
+
+/**
+ * Wire native-menu IPC channels to the command store.
+ * Channel names match what main.js dispatches via webContents.send.
+ */
+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');
+  useMenuAction('undo', 'editor.undo');
+  useMenuAction('redo', 'editor.redo');
+  useMenuAction('adjust-font-size', 'font.size', (direction) => direction as string);
+  useMenuAction('load-custom-css', 'theme.loadCustomCss');
+  useMenuAction('clear-custom-css', 'theme.clearCustomCss');
+  useMenuAction('load-template-menu', 'template.load', (name) => name as string);
+  useMenuAction('print-preview', 'print.preview');
+  useMenuAction('print-preview-styled', 'print.previewStyled');
+  useMenuAction('file-opened', 'file.opened', (payload) => payload);
+  useMenuAction('clear-recent-files', 'file.clearRecent');
+}