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/index.html

@@ -1925,7 +1925,7 @@
                     <label class="set-chip" data-preview="header" data-preview-order="9"><input type="checkbox" id="appSettingsShowAttachmentsButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="m21.44 11.05-9.19 9.19a6 6 0 0 1-8.49-8.49l9.19-9.19a4 4 0 0 1 5.66 5.66l-9.2 9.19a2 2 0 0 1-2.83-2.83l8.49-8.48"/></svg><span>Attachments</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="10"><input type="checkbox" id="appSettingsShowFileViewerButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M3 7a2 2 0 0 1 2-2h4l2 2h8a2 2 0 0 1 2 2v8a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2z"/></svg><span>File Viewer</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="11"><input type="checkbox" id="appSettingsShowMultiMonitorButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="4" width="13" height="9" rx="1.5"/><rect x="11" y="9" width="11" height="8" rx="1.5"/></svg><span>Multi-monitor</span></label>
-                    <label class="set-chip" data-preview="header" data-preview-order="15"><input type="checkbox" id="appSettingsShowSplitButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="3" width="20" height="18" rx="2"/><line x1="12" y1="3" x2="12" y2="21"/></svg><span>Split</span></label>
+                    <label class="set-chip" data-preview="header" data-preview-order="11.5"><input type="checkbox" id="appSettingsShowSplitButton"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><rect x="2" y="3" width="20" height="18" rx="2"/><line x1="12" y1="3" x2="12" y2="21"/></svg><span>Split</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="13" data-preview-text="42%"><input type="checkbox" id="appSettingsShowPlanUsageLimits"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M4 18a8 8 0 1 1 16 0"/><path d="M12 18l4.5-5"/></svg><span>Plan Usage</span></label>
                     <label class="set-chip" data-preview="header" data-preview-order="14"><input type="checkbox" id="appSettingsShowLifecycleLog"><svg class="set-chip-ico" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.9" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z"/><polyline points="14 2 14 8 20 8"/><line x1="16" y1="13" x2="8" y2="13"/><line x1="16" y1="17" x2="8" y2="17"/></svg><span>Lifecycle Log</span></label>
                   </div>