feat(tiles): tile chords in the shortcut registry, and the grid never beside a split

Shortcut registry (DEFAULT_SHORTCUTS, group Tiles, all rebindable):

- Toggle Tile Grid, Ctrl+Shift+G: opens the grid this tab last left (one step
  back after a selection outside it), else an open split as two tiles, else
  the active session as one tile; pressed again, back to the single view of
  the focused session. xterm emits nothing for a shifted Ctrl letter; the
  browser's find-previous is overridden only where the grid can open.
- Focus Tile Left/Right/Up/Down, Alt+Shift+Arrows: a human selection of the
  tile in that direction.
- Remove Focused Tile, unbound: the session keeps running.

tileShortcutFor() decides whether a chord applies (the toggle wherever a grid
could open, the rest only while one is open, so outside the grid
Alt+Shift+Arrows reach the terminal untouched) and is registry-aware. The
capture handler dispatches it, and the main terminal's and every tile's xterm
key handler return false for it, for every event type and before the
Shift+Enter gate, so a chord that applies never reaches a PTY.

Coexistence with the split pane: opening the grid over an open split closes
it (no wasted resize for the pane about to park) and seeds the grid with both
of its sessions, Pane A focused. While the grid is open openSplitPicker and
openSplitPane refuse and the Split button reads as unavailable
(aria-disabled); closing the grid never reopens a split.

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

src/web/public/styles.css

@@ -2510,6 +2510,12 @@ html[data-tab-orientation='vertical'].tab-rail-compact
   display: none !important;
 }
 
+/* The tile grid is open (tile-grid.js): the split cannot open beside it. */
+.btn-split.btn-split--blocked {
+  opacity: 0.4;
+  cursor: not-allowed;
+}
+
 /* While a split is open, the button's own click closes it instead of opening
    the picker — the accent fill is what tells the user which of its two
    behaviours the next click will get (aria-pressed carries the same state