feat(tiles): Move Tile Left/Right/Up/Down (Ctrl+Shift+Arrows)

Four rebindable registry chords in the Tiles group move the focused
tile: it trades places with the neighbour the Alt+Shift+Arrow focus
chords pick (tileInDirection), through the same _reorderTiles path as
the header drag, and keeps the focus. Nothing at an edge.

They go through tileShortcutFor()/runTileShortcut() like the other
tile chords, so every xterm key handler swallows them while they
apply: only while the grid is open, a zoomed grid included (a no-op
there, so the keys never reach the CLI), and never in a text field
other than xterm's own textarea, where Ctrl+Shift+Arrows select by
word.

Ctrl+Shift+Arrows because every other two-modifier arrow chord is
taken: Ctrl+Alt switches workspaces (GNOME, Xfce, some Windows
graphics drivers), Ctrl+Alt+Shift moves a window to another workspace
(GNOME, Cinnamon, Xfce), Super belongs to the desktop, Alt is the
browser's back and forward, Alt+Shift focuses tiles. No browser,
GNOME, KDE, macOS or Claude Code default uses Ctrl+Shift+Arrows; it
costs a terminal editor's word selection inside a tile while the grid
is open.

The Help modal lists the chords and the header drag; the shortcut
overlay lists the registry. zh-CN entries for every new string.

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

src/web/public/index.html

@@ -838,6 +838,8 @@
             <div class="shortcuts-grid">
               <div><kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>G</kbd></div><div>Toggle Tile Grid</div>
               <div><kbd>Alt/Option</kbd>+<kbd>Shift</kbd>+<kbd>Arrows</kbd></div><div>Focus Tile Left / Right / Up / Down</div>
+              <div><kbd>Ctrl</kbd>+<kbd>Shift</kbd>+<kbd>Arrows</kbd></div><div>Move Tile Left / Right / Up / Down</div>
+              <div><kbd>Drag</kbd> a tile's header</div><div>Move the Tile (onto Another: Swap)</div>
               <div><kbd>Alt/Option</kbd>+<kbd>Shift</kbd>+<kbd>Enter</kbd></div><div>Zoom Focused Tile</div>
               <div><kbd>Ctrl/Cmd</kbd>+<kbd>Click</kbd> a tab</div><div>Add the Session to the Tile Grid</div>
               <div><kbd>Right-click</kbd> the Tiles button</div><div>Choose Which Sessions to Tile</div>