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-select-branch.test.ts

@@ -201,6 +201,35 @@ describe('moving focus between tiles', () => {
     expect(app.markIdleAlertSeen).not.toHaveBeenCalled();
   });
 
+  // The last tile popped out closes the grid. The single view must never try
+  // to show that session: selectSession only raises its window, and the parked
+  // terminal's stale pre-grid content would stay up under its tab (and be
+  // saved as its snapshot on the next switch).
+  it('popping out the LAST tile falls through to the next session, never the popped-out one', () => {
+    const app = makeGridApp(['s-a']);
+    app.openTileGrid(['s-a']);
+    app.$ = () => null;
+    app.selectSession = vi.fn();
+    app._markDetached('s-a', true);
+    expect(app._tilesOwnTerminal()).toBe(false);
+    expect(app.selectSession.mock.calls).toEqual([['s-other', { auto: true }]]);
+    // Nulled before the selection, so the parked buffer is not snapshotted as s-a's.
+    expect(app.activeSessionId).toBeNull();
+  });
+
+  it('popping out the last tile with every other session popped out too: the welcome screen, the parked terminal cleared', () => {
+    const app = makeGridApp(['s-a']);
+    app.detachedSessions.add('s-other');
+    app.openTileGrid(['s-a']);
+    app.$ = () => null;
+    app.selectSession = vi.fn();
+    app._markDetached('s-a', true);
+    expect(app.selectSession).not.toHaveBeenCalled();
+    expect(app.activeSessionId).toBeNull();
+    expect(app.terminal.clear).toHaveBeenCalled();
+    expect(app.showWelcome).toHaveBeenCalled();
+  });
+
   it('going Home leaves the grid, remembered', () => {
     const app = openGrid();
     app.goHome();