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>

docs/wiki/Keyboard-Shortcuts.md

@@ -10,7 +10,6 @@ Press `Ctrl+?` in the app for the same list in a floating overlay.
 | ------------------------------- | --------------------------------------------------------------- |
 | `Ctrl+K` (also `Cmd+K`, `Alt+K`)| Find an open session or start a new one.                         |
 | `Ctrl+Tab`                      | Next session.                                                    |
-| `Alt+←` / `Alt+→`               | File Viewer window: back / forward between previewed files.      |
 | `Alt+[` / `Alt+]`               | Previous / next tab.                                             |
 | `Alt+1` to `Alt+9`              | Switch to tab N. Physical keys, so macOS Option layouts work.    |
 | `Ctrl+Shift+{` / `Ctrl+Shift+}` | Move the active tab left / right.                                |
@@ -58,11 +57,12 @@ default. See [Tile Grid](Tile-Grid).
 
 ## Everything else
 
-| Shortcut       | Action                          |
-| -------------- | ------------------------------- |
-| `Ctrl+Shift+V` | Toggle voice input.              |
-| `Ctrl+?`       | Shortcut reference overlay.      |
-| `Escape`       | Close panels and modals.         |
+| Shortcut          | Action                                                       |
+| ----------------- | ------------------------------------------------------------ |
+| `Ctrl+Shift+V`    | Toggle voice input.                                          |
+| `Ctrl+?`          | Shortcut reference overlay.                                  |
+| `Escape`          | Close panels and modals.                                     |
+| `Alt+←` / `Alt+→` | File Viewer window: back / forward between previewed files.  |
 
 ## Rebinding