feat(tiles): zoom a tile (button, Alt+Shift+Enter), and auto-zoom when the window is too small

⤢ in a tile's header, or Alt+Shift+Enter (registry entry zoom-tile, applies
only while the grid is open and is swallowed before the Shift+Enter newline
gate), makes that tile fill the grid like tmux zoom. The other tiles stay
connected but hidden, so they measure nothing and send no resize; pressing it
again restores the grid and refits every tile, since the hidden ones have a
stale size. Zooming a tile that is not focused focuses it first (a human
selection).

As in tmux, moving focus to another tile restores the grid, and so does
removing the zoomed tile or adding one while a tile is zoomed by hand.

When the grid area cannot fit the tiles' minimum size, the grid zooms the
focused tile itself with a hint; that zoom follows focus and lifts once the
window fits again. A zoom the user chose is left alone.

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

src/web/public/styles.css

@@ -19486,6 +19486,17 @@ html[data-session-list="sidebar"][data-sidebar="collapsed"] .btn-sidebar-toggle
   outline: none;
 }
 
+/* Zoom (tmux-style): one tile fills the grid; the others stay connected but
+   hidden, so they measure nothing and send no resize. */
+.tile-grid.tile-grid--zoomed .tile:not(.tile--zoomed) {
+  display: none;
+}
+
+.tile-btn.tile-zoom[aria-pressed='true'] {
+  opacity: 1;
+  color: var(--accent, #4a9eff);
+}
+
 /* A permission prompt or question, visible across the room. Only the border
    colour and a box-shadow animate: anything that moves layout would resize
    the tile's PTY every frame. */