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/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