docs(file-viewer): list Alt+Left/Right in the shortcut overlay and help

Merge-time fix for #659.

- Alt+Left/Right (File Viewer back/forward) is a display-only entry in
  DEFAULT_SHORTCUTS, so the Ctrl+? overlay lists it, and a row in the
  help modal's Panels group, pinned in help-modal-shortcuts.test.ts.
  zh-CN entry for the new label; the "Restore size" entry is dropped,
  since nothing shows that string any more.
- The wiki row moves from "Sessions and tabs" to "Everything else".
- The grip comment in styles.css says only phones hide the grip;
  tablets show and use it.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/help-modal-shortcuts.test.ts

@@ -51,6 +51,7 @@ describe('help modal shortcuts', () => {
     expect(helpModal).not.toMatch(/Ctrl<\/kbd>\s*\+\s*<kbd>\/<\/kbd>.*?Show Shortcuts/i);
     expectShortcut(helpModal, ['Ctrl', 'Shift', 'V'], 'Voice Input');
     expectShortcut(helpModal, ['Escape'], 'Close Panels');
+    expectShortcut(helpModal, ['Alt/Option', 'ArrowLeft'], 'File Viewer Back / Forward');
   });
 
   it('documents the tile grid chords', () => {