feat(tiles): no + in the tile header (owner decision 9)

Owner: "remove the + button from these views". The tile header is now
● name ... ⋯ ⤢ ×. Gone with it, because nothing else used them: the +
menu (openTileAddMenu, closeTileAddMenu and their hooks in closeTileGrid
and the global Escape handler), its "New session in this case" entry and
runInCaseForTiles, the .tile-add-empty rules, the four i18n entries only
the menu showed, and buildTilePickerSessions' exclude argument (only the
menu passed it).

Every other way of adding tiles stays and needed nothing from the menu:
the Tiles button and its right-click picker, Ctrl/Cmd+click on a tab,
dragging a tab onto a tile or an empty slot, "Open group as tiles", and
Run joining the open grid (_joinTileGridFromRun). The picker list, the
cap helper _tileGridLimit and the user-text skip on names are shared and
kept.

Tests: the + menu cases (picker, cap, auto-join, the zh-CN harvest) are
removed; tile-grid-chrome pins the header as exactly ⋯ ⤢ × with no add
menu or runInCaseForTiles left on the app.

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

src/web/public/i18n.js

@@ -129,10 +129,6 @@
     'Open tiles': '打开平铺',
     'No sessions to show as tiles': '没有可平铺显示的会话',
     'This group has no session to show as tiles': '此分组没有可平铺显示的会话',
-    'Add a session to the grid': '向平铺网格添加会话',
-    'Every open session is already tiled': '所有打开的会话都已平铺',
-    'New session in this case': '在此案例中新建会话',
-    'This session is not in a case': '此会话不属于任何案例',
     'Zoom this tile': '放大此窗格',
     'Restore the grid': '恢复平铺网格',
     'Remove tile (the session keeps running)': '移除窗格（会话继续运行）',