fix(file-viewer): phones ignore expand and the grip, folder paging follows history, swallow Alt+arrows

Under 600px the window stays full screen whatever Expand or a drag says, and both
controls are hidden. Stepping onto a folder-grid entry goes through the grid so its
index follows. Alt+left/right is swallowed at the ends of the list too. Adds tests
for the key handler, the grip clamp and the history/grid case, zh-CN titles and wiki lines.

docs/wiki/Keyboard-Shortcuts.md

@@ -10,6 +10,7 @@ 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.                                |

docs/wiki/Working-With-Files.md

@@ -27,6 +27,11 @@ stream rather than server memory). Sensitive paths (`.env`, anything
 matching credentials, `~/.ssh`, AWS credentials) are blocked from download, and SVG and HTML
 are served as downloads rather than rendered, so they cannot execute in the page.
 
+The preview window has **Back** and **Forward** buttons (also `Alt+←` and `Alt+→`) once you
+have opened a second file, so a link into another document is no longer one way. **Expand**
+fills the screen and is remembered per device, and the corner grip resizes the window. On a
+phone the window is always full screen, so Expand and the grip are hidden there.
+
 Closing the preview pauses and unloads any playing media. A video that keeps playing after
 you close the panel means you are on an old version.