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/tile-grid-i18n.test.ts

@@ -215,6 +215,14 @@ async function exercise() {
   harvestAll(app, 'zoomed');
   app.zoomTile('s-1');
 
+  // A file that is not an image dropped on a tile.
+  section.dispatch('drop', {
+    target: section.children.find((el) => el.dataset.sessionId === 's-1'),
+    dataTransfer: { types: ['Files'], files: [{ type: 'application/pdf' }] },
+    preventDefault: vi.fn(),
+  });
+  harvestAll(app, 'file drop');
+
   // The toasts of a full grid, by the cap and by the window.
   app._joinTileGridFromRun('s-7');
   app.addSessionToTiles('s-7');
@@ -314,6 +322,7 @@ describe('every tile grid string the code puts on screen translates to zh-CN', (
       'Could not attach the session',
       'This group has no session to show as tiles',
       'No sessions to show as tiles',
+      'Only image files are supported',
       'Split: unavailable while tiles are open',
       'Tiles: show several sessions side by side (right-click for how many)',
       'Tiles: back to a single session (right-click for how many tiles)',