feat(i18n): the tile grid in Chinese (zh-CN)

Owner request: with App Settings language set to 简体中文, the grid reads
fully in Chinese. Every string the grid puts on screen gets its own
ZH_CN entry, so none reaches the generic leading-verb fallback: the Tiles
button (both states, with the right-click hint), the Tiles and Split
chips, the grid region, the Help modal's Tiles rows, the shortcut
registry's Tiles group and labels (overlay and App Settings list, and
"not bound"), "Open group as tiles", the picker, a tile's +, the header
buttons, the Attach overlay (not attached, attaching, exited, ended, the
hint), the empty slot, the dividers, the Split button while tiles are
open, and the toasts. Strings with a count, an exit code or a duration
are translateDynamic patterns: the cap texts (both wordings, with and
without ": the new session opens on its own"), "This window fits N
tile(s)", the auto-zoom hint, "The agent exited (N)" / "(signal N)", the
crash-restart confirm (the existing confirm wrapper runs it through t();
the session name passes through untranslated, in the single view too),
and the tile header tooltip ("idle 3m"), which requires the duration so
bare state words stay out of the table (they collide with other
surfaces, see mobile-overview.js).

Wording: 平铺 for the feature, 窗格 for one tile, 附加 for attach, 智能体,
案例, as the table already has them. Key names stay; Click, Right-click
(mouse actions) and Arrows in the Help modal's key column are translated.
English reads exactly as before (only additions to the table).

test/tile-grid-i18n.test.ts drives the real tile code through every
state that writes text, harvests each string and requires Chinese with
no Latin word left beyond key names and durations, and the same English
in en; plus the markup through the real translator in JSDOM, and session
and group names (also when they equal a UI word) staying untranslated.

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

docs/architecture-invariants.md

@@ -809,6 +809,8 @@ Further detail: with many sessions the horizontal strip stops being scannable, w
 
 ⚠️ **Opening.** The Tiles button's click and `Ctrl+Shift+G` are one function, `toggleTileGrid` (owner decision 8), so the two cannot drift. It opens the grid at once on `tileGridOpenSet` (constants.js, pure): (a) the stored grid if any of its sessions survive, opened EXACTLY (`openTileGrid(..., { mergeSplit: false })`: an open split closes and its sessions do not join), (b) else an open split's two sessions, (c) else the picker's list (tab order, no detached sessions) up to `_tileGridLimit().capacity`, the active session always included and focused. Ctrl/Cmd+click with the grid closed opens the same set plus the clicked session. The picker is on right-click (`oncontextmenu`, which calls `preventDefault` for the browser menu); with the grid open it is preselected with the current tiles and Open REPLACES them (close without reselect, null `activeSessionId`, reopen, as "Open group as tiles" does).
 
+⚠️ **zh-CN.** Every string the grid puts on screen has its own entry in i18n.js's `ZH_CN` (or a `translateDynamic` pattern for counts, exit codes and the header tooltip's state plus duration, which requires the duration: bare state words stay out of the table, see mobile-overview.js), so nothing reaches the generic leading-verb fallback. `test/tile-grid-i18n.test.ts` drives the real tile code through every state that writes text, harvests each string and requires a full translation (and the same English back in `en`); a new tile string needs its entry or that test fails. Session and group names stay user text (`data-i18n-skip`). ⚠️ A refresh that skips unchanged text must compare with the last ENGLISH value it set (`entry.headerLabel`, `entry.overlayLabel`, `entry.zoomLabel`), never the DOM: in zh-CN the DOM holds the translation, so a DOM compare rewrites English on every `session:updated` for the observer to translate again.
+
 ⚠️ **Chrome.** The header is a fixed 28px sibling of the body (its buttons are 26px targets with 16 to 19px glyphs, the app header's own icon-button size), refreshed in place on every tab render (`_renderSessionTabsImmediate` wrapper), never by rewriting the tile (that would take its xterm along) and never growing (#464: the body holds the xterm). Header buttons stop pointerdown, so acting on an unfocused tile does not focus it. × removes the tile only; killing stays behind the session menu's Close session. The `needs` pulse animates `box-shadow` only. Dividers are their own 6px grid tracks (`gap: 0`), tiles placed explicitly; a drag uses pointer capture, reflows the affected tiles locally per animation frame and sends ONE `fit()` (one PTY resize) per affected tile at pointer-up; closing the grid or removing a tile mid-drag tears it down. A tab dragged onto a tile replaces it (or swaps two tiles); tiles and empty slots handle `dragover`/`drop` in the CAPTURE phase and stop it, because the drag carries the session id as text and xterm's helper textarea would type it into the PTY.
 
 ⚠️ **Attach.** A tile whose session has no PTY (`pid === null`) or whose socket closed because it exited (4009) shows an Attach overlay (absolute, the body keeps its size): `POST /interactive` (or `/shell`) with NO body, one in flight per session, a tripped PTY-exit breaker only through the same confirm as the single view, then the tile is remounted (a stopped socket cannot reconnect). The routes report a refusal in the ENVELOPE of a 200, so the response body is read, not `res.ok`. An agent that exited in a live pane (`paneExit`) cannot be started again in place (both routes refuse while the pane's tmux client runs): its tile shows the exit and points at Close session.

docs/tile-grid-plan.md

@@ -32,6 +32,10 @@ or settled a question the spec left open. The invariants as built are in
 - **Tile header buttons are 26px targets with 16 to 19px glyphs** (owner feedback: the
   first build's 12px glyphs read as tiny next to the name), the size of the app header's own
   icon buttons; the header grew from 24 to 28px to hold them.
+- **The grid is translated for 简体中文 (zh-CN)** (owner request): 平铺 for the feature, 窗格 for
+  one tile, key names untranslated, mouse actions in the Help modal's key column (`Click`,
+  `Right-click`) and `Arrows` translated. Every string has its own entry or pattern; refreshes
+  compare with the last English value, not the translated DOM.
 - **The Tiles button opens the grid at once** (owner decision 8): a click (and
   `Ctrl+Shift+G`, the same `toggleTileGrid`) opens `tileGridOpenSet` (constants.js): the
   grid this tab last had, else an open split's two sessions, else the open sessions in tab