feat(web): optional collapsible left session sidebar

The header tab strip stops working past roughly a dozen sessions: it wraps
into two or three rows, eats vertical space and still cannot be scanned.
This adds a vertical session list in a left <aside> as an ALTERNATIVE
layout — a filter box, a live count, and a 44px collapsed rail that keeps
the ambient signal (status dot, task badge) visible.

The strip is not removed. Settings -> Display -> Tab Bar -> Session List
Layout switches between them and the default stays 'header', so existing
users see no change until they opt in.

Structure: one #sessionTabs element, two mount points. applySessionListLayout()
re-parents the SAME node between #sessionTabsHost and #sessionSidebarList,
which is why there is no second renderer and no duplicated wiring — app.$()
caches getElementById results and never invalidates them, so a moved node
keeps every existing consumer (settings-ui, webview-tabs, the generated
gesture bundle, the mobile tests) working untouched.

Notable integration points:
- Below 1024px the sidebar is an off-canvas drawer overlaying the terminal;
  closed it gets inert + aria-hidden so it cannot be tabbed into, and touch
  swipes over it no longer switch sessions.
- Subagent and ultracode windows anchor to the right edge of a sidebar row
  instead of its bottom, connector curves follow.
- Alt+B toggles; the chord is gated out of the PTY so xterm cannot also
  write ESC b into a live session.
- Collapse state lives in its own localStorage key (the settings blob is
  rebuilt from DOM controls on every save) and falls back to in-memory
  intent where storage throws.

Verified: frontend syntax + public asset checks, tsc, eslint, 26 new jsdom
tests, and a headless-Chromium harness (scripts/verify-session-sidebar.mts)
that renders a synthetic 25-session fleet in both layouts at 1600/1000/393px
and asserts mount point, widths, inert/aria state and row count.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/i18n.js

@@ -48,6 +48,10 @@
     'Skip to terminal': '跳转到终端',
     'Go to main page': '返回主页',
     'Session tabs': '会话标签页',
+    /* 'Sessions' (the sidebar heading) is already mapped further down. */
+    'Collapse session sidebar': '收起会话侧边栏',
+    'Expand session sidebar': '展开会话侧边栏',
+    'Filter sessions': '筛选会话',
     'Admin Panel': '管理面板',
     'Open admin panel': '打开管理面板',
     'Re-dock to dashboard (close window)': '重新停靠到主界面（关闭窗口）',
@@ -226,6 +230,11 @@
     'Cron Button': '定时任务按钮',
     'Redraw Terminal Button': '重绘终端按钮',
     'Tab Bar': '标签栏',
+    'Session List Layout': '会话列表布局',
+    'Header tab strip': '顶栏标签条',
+    'Left sidebar': '左侧边栏',
+    'Show the session list as a horizontal strip in the header, or as a collapsible sidebar on the left':
+      '将会话列表显示为顶栏中的横向标签条，或左侧的可折叠侧边栏',
     'Tall Tabs (Name + Folder)': '双行标签（名称 + 文件夹）',
     Panels: '面板',
     Monitor: '监视器',