fix(tiles): file drops, focus handoffs, pop-out fallback, split merge, tablet default

- A file dragged onto the tile grid navigated the browser away: the single
  view's drop handler sits on #terminalContainer, hidden while tiles are
  open. The #tileGrid section now cancels every file dragover and drop
  (bubble phase, so tab and tile drags stay with _acceptTabDrops), and a
  drop on a tile uploads its images to THAT tile's session through
  _uploadAndInsertImages, with the same "Only image files are supported"
  toast as image-input.js (now in the zh-CN table).
- App-driven refocus no longer moves DOM focus into another session's
  xterm: a remote delete of the focused tile, _reconcileTileGrid and a
  socket closed with 4003/4004/4010 (_onTileExit) pass focus: false.
  removeTile gains a focus option; user-initiated removes keep focusing.
- Popping out the last tile left the parked terminal's stale content under
  the popped-out tab (and snapshotted it on the next switch).
  _selectAfterTileGrid treats a detached session as unusable for both the
  focused id and the fallback.
- "Open group as tiles" and Ctrl/Cmd+click with the grid closed pass
  mergeSplit: false, so an open split no longer adds its two sessions on
  top of a set already sized to the group, the count and the window.
- Touch-primary devices (primary pointer coarse: iPad, Android tablets)
  default the Tiles button OFF in getDefaultSettings(); touchscreen
  laptops (fine primary pointer) keep the desktop default ON. The button,
  the App Settings chip and the Ctrl+Shift+G gate all resolve an absent key
  through these defaults, so they agree. CLAUDE.md and the invariants doc
  say so.

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

test/mocks/tile-grid-vm.ts

@@ -375,6 +375,10 @@ export function resetGridHarness() {
   // The CLI catalog the server injects (labels for the harness logos); a test sets its own.
   delete windowStub.__codemanCliCatalog;
   section.children = [];
+  // The grid binds its file-drop guard on the section once per app; an earlier
+  // test's app must not still be listening there.
+  section.listeners = {};
+  section.captureFlags = {};
   main.className = 'main';
   // A split a test left open moved .terminal-wrap into its container, with
   // Pane A's header strip in it.