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>

docs/architecture-invariants.md

@@ -805,7 +805,7 @@ Further detail: with many sessions the horizontal strip stops being scannable, w
 
 ### Tile grid
 
-**Tile grid** (`showTileGridButton`, header Tiles button + `Ctrl+Shift+G`, default ON on desktop and OFF on handhelds, per-device: in `displayKeys`, stripped from the settings PUT, not in `SettingsUpdateSchema`; desktop-only at 1180px by a JS width check with a live media listener plus a `@media (max-width: 1179px)` backstop, never in a solo window). 1 to 6 live sessions side by side in one window, each a `TerminalTile` (terminal-tile.js), orchestrated by tile-grid.js (load order 7.6, `CodemanApp.prototype` methods like the split's). Pure helpers live in constants.js as `window.CodemanTileGrid`: `computeTileLayout` (1x1, 2x1, 3x1 on a grid area at least 1800px wide else 2x2, 2x2, 3x2, and 3x3 up to `TILE_LAYOUT_MAX` (9), unreachable today; `fits` against a 480x240 minimum tile), `tileGridCapacity` (never more than `TILE_GRID_MAX`), `sanitizeTileGridState` (truncates a stored grid to the cap, keeps focus only if it survives), `buildTilePickerSessions`, `dragTrackFractions`, `tileNeighbor`, `tileInDirection`, `cycleTile`, `TILE_SCROLLBACK` (10,000), and `TILE_GRID_MAX` (6), the ONE cap (owner decision 7: six tested smooth on a real desktop, nine missed the headless frame bar). ⚠️ Every limit reads the cap through `_tileGridLimit()` (tile-grid.js: the window's capacity, at most the cap), never a literal, and its texts say which binds ("at most 6 tiles" vs "what this window fits"). The grid is a `<section class="tile-grid">` SIBLING of `.terminal-wrap`, swapped in by `.main.tiles-active` (no reparenting); `.main.webview-active .tile-grid` hides it like the single view.
+**Tile grid** (`showTileGridButton`, header Tiles button + `Ctrl+Shift+G`, default ON on desktop and OFF on handhelds and touch-primary tablets (primary pointer coarse, resolved in `getDefaultSettings()`, never `isTouchDevice()`, so a touchscreen laptop keeps ON), per-device: in `displayKeys`, stripped from the settings PUT, not in `SettingsUpdateSchema`; desktop-only at 1180px by a JS width check with a live media listener plus a `@media (max-width: 1179px)` backstop, never in a solo window). 1 to 6 live sessions side by side in one window, each a `TerminalTile` (terminal-tile.js), orchestrated by tile-grid.js (load order 7.6, `CodemanApp.prototype` methods like the split's). Pure helpers live in constants.js as `window.CodemanTileGrid`: `computeTileLayout` (1x1, 2x1, 3x1 on a grid area at least 1800px wide else 2x2, 2x2, 3x2, and 3x3 up to `TILE_LAYOUT_MAX` (9), unreachable today; `fits` against a 480x240 minimum tile), `tileGridCapacity` (never more than `TILE_GRID_MAX`), `sanitizeTileGridState` (truncates a stored grid to the cap, keeps focus only if it survives), `buildTilePickerSessions`, `dragTrackFractions`, `tileNeighbor`, `tileInDirection`, `cycleTile`, `TILE_SCROLLBACK` (10,000), and `TILE_GRID_MAX` (6), the ONE cap (owner decision 7: six tested smooth on a real desktop, nine missed the headless frame bar). ⚠️ Every limit reads the cap through `_tileGridLimit()` (tile-grid.js: the window's capacity, at most the cap), never a literal, and its texts say which binds ("at most 6 tiles" vs "what this window fits"). The grid is a `<section class="tile-grid">` SIBLING of `.terminal-wrap`, swapped in by `.main.tiles-active` (no reparenting); `.main.webview-active .tile-grid` hides it like the single view.
 
 ⚠️ **The main terminal is parked while the grid is open.** Opening runs `_cleanupPreviousSession()` ONCE (its snapshot is right at that moment, and it closes the main socket), and `activeSessionId` is always the FOCUSED tile's session, so everything keyed on it (files panel, git status, respawn and Ralph panels, subagent windows, voice, image paste, the tab highlight) follows focus. With the main socket closed `_wsReady` is false, so every SSE terminal handler would write the focused tile's output into the hidden xterm: `_tilesOwnTerminal()` turns `_onSessionTerminal`, `_onSessionClearTerminal`, `_onSessionNeedsRefresh` (returns `false`, which the drop recovery reads), `_scheduleDroppedOutputRecovery`, the `writeln` in `_onSessionCompletion`/`_onSessionError`, `sendResize`, `throttledResize`'s fit and `_maybeRefetchFullHistory` into no-ops; `retryConnection` and `handleInit` re-arm the TILES' sockets instead of the main one (`handleInit` keeps live tiles, drops dead ones through `_reconcileTileGrid`, and re-selects only when the focused tile is gone, so an SSE blip never hides an open web tab). ⚠️ The page's SSE filter names `TILE_GRID_SSE_FILTER` (constants.js, an id no session takes) instead of the focused session while tiles own the terminal: the server's filter gates only `session:terminal` batches, which the parked terminal could only parse and drop. Both places that set the filter ask `_sseFilterSessionId()`: the live re-subscribe every tile focus runs (`_updateSseSubscription`) and the connect URL an SSE reconnect rebuilds (`connectSSE`); leaving the grid re-subscribes the shown session through `selectSession`. `test/sse-tile-grid-filter.test.ts` pins the server's side in multi-user mode (the id is accepted on connect and on re-subscribe, and session/hook events still reach their owner through it). ⚠️ The WebGL long-task observer watches the WHOLE page: it counts nothing while tiles own the terminal, or tile renders would write the sticky 7-day WebGL disable. The header connection dot reads the tile sockets (`_tileGridSocketState`; a tile stopped for good does not count). `_focusedPane()` answers with the focused tile even when DOM focus left every terminal, and `_forEachTile` reaches every grid tile (`{ grid: false }` skips them where tiles keep their own font size). Leaving the grid destroys every tile, resets `_lastResizeDims`, invalidates the main terminal's cached content for EVERY tiled id (`_xtermSnapshots`, `codeman-xs-<id>`, `terminalBufferCache`: written before the grid opened, and selectSession paints a snapshot as its first frame) and replays the focused session through `selectSession(id, { forceReload: true, auto: true })`.