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/terminal-split.js

@@ -46,6 +46,9 @@ Object.assign(CodemanApp.prototype, {
     // the old exact-node check below) bubbled straight through to
     // `document` and self-closed the menu it just opened.
     event?.stopPropagation();
+    // The tile grid and the split are never open together (tile-grid.js); the
+    // Split button shows as unavailable meanwhile.
+    if (this._tilesOwnTerminal?.()) return;
     if (this._splitPane) {
       this.closeSplitPane();
       return;
@@ -139,6 +142,8 @@ Object.assign(CodemanApp.prototype, {
     // see _applySplitButtonVisibility's comment for why both a JS check and
     // a CSS backstop exist.
     if (window.innerWidth < SPLIT_PANE_MIN_WIDTH) return;
+    // Never beside the tile grid: the main terminal is parked while it is open.
+    if (this._tilesOwnTerminal?.()) return;
     // No active session means there is no `.terminal-wrap` to split against
     // (the welcome overlay is showing) — without this, a split opened from
     // the home screen still created the container and connected Pane B, just