fix(split-pane): gate app-level chords out of Pane B, address Ark0N's third pass

Pane B had no attachCustomKeyEventHandler of its own, so the document
capture-phase shortcut handler's preventDefault() (which does not stop
xterm) left Ctrl+K/Alt+1/Alt+B ALSO writing their raw byte/escape
sequence into Pane B's live PTY on top of whatever the app action did
to Pane A. Pane B now installs the same registry-aware gates the
primary pane's own attachCustomKeyEventHandler uses. Ctrl+V is left on
xterm's default paste — no image-paste trap to route it to.

Plus the rest of the review's smaller items:
- Narrowing the window past the desktop gate now closes an open split
  instead of leaving it stranded on screen.
- Split is refused while a web tab is active (activeWebviewId), which
  used to open Pane B's socket behind a hidden container.
- Pane B now handles the server's `{t:'r'}` refresh frame via a shared
  _loadBuffer() helper (also used by connect()), instead of ignoring it.
- The divider drag now uses pointer events + setPointerCapture (mirrors
  tab-rail-resize.js), a button!==0 guard, preventDefault, and a
  body.split-pane-resizing cursor/selection lock — a plain mousedown
  drag selected the text under the cursor as it crossed both terminals.
- Pane B's close control and the picker rows are real <button>s now
  (keyboard-reachable), with matching CSS chrome resets.
- Dropped the redundant CodemanBase.base prefix on the buffer fetch
  (the global fetch wrapper already applies it).
- data-preview-order for the Split settings chip moved from a collision
  with Ultracode Agents (both 15/12) to 11.5, matching its real
  position between Multi-monitor and Ultracode Agents in the header;
  widened test/app-settings-structure.test.ts's regex to allow the
  decimal (Number() already parses it fine for the preview sort).
- Added zh-CN i18n entries for the Split button and empty-picker text.
- Dropped the stray unused `vi` import Ark0N flagged as unrelated to
  this feature (vitest's `globals: true` makes it ambient anyway).
- Documented the fix and the deliberate no-cid/seq choice in the
  split-pane-sessions architecture-invariants entry.

Added a real-Chromium regression test asserting Ctrl+K/Alt+1/Alt+B
dispatched at Pane B's own textarea send no `{t:'i'}` frame over its
WebSocket. Full CI gate green (409 files, 7736 tests) plus all 8
split-pane browser tests.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/web/public/i18n.js

@@ -73,6 +73,10 @@
     'File Viewer': '文件查看器',
     'Open file viewer': '打开文件查看器',
     'Open Codeman across all displays': '在所有显示器上打开 {name}',
+    'Split: open a second session beside this one': '分屏：在旁边打开第二个会话',
+    'Split: close the second session': '分屏：关闭第二个会话',
+    'Close split': '关闭分屏',
+    'No other sessions to split with': '没有其他可用于分屏的会话',
     'Ultracode / Workflow agents': 'Ultracode / Workflow 智能体',
     'Open ultracode workflow agents': '打开 Ultracode 工作流智能体',
     Notifications: '通知',