Merge christianhaberl's session-sidebar branch, ported to current master

Brings in https://github.com/christianhaberl/Codeman/pull/4 (three commits,
authorship preserved) and adapts it across the 211 commits master gained
since the branch was cut:

- App Settings control re-authored for the set-* surface (PR #278): a
  set-row in Layout -> Tabs, replacing the old settings-item markup the
  branch targeted. i18n description synced.
- Lineage arcs (PR #291, post-branch) are SKIPPED in sidebar layout:
  computeLineagePath()'s U-bridge geometry hangs from the horizontal
  strip's bottom edge and has no meaning against a vertical list. The
  lineage strip-scroll listener now also redraws subagent/ultracode
  connectors while the sidebar scrolls vertically.
- The desktop home tab rail (post-branch) defers to the sidebar: both dock
  the session list flush left, and the rail would render z-ordered under it.
- Active-row reveal unified into _scrollActiveTabIntoView() (#257 landed on
  master after the branch): sidebar mode branches to scrollIntoView
  block:'nearest', and _fullRenderSessionTabs() restores scrollTop alongside
  the #257 scrollLeft restore so ambient rebuilds cannot yank a mid-scroll
  sidebar back to the top.
- Mobile active-tab hoisting the branch guarded against no longer exists on
  master (removed by #257); kept master's order-stable render.

Verified: typecheck, lint, format:check, check:frontend-syntax,
check:public-assets, PostCSS parse of both merged stylesheets, the 26 new
jsdom tests, the structural guard suites, and the headless-Chromium harness
(scripts/verify-session-sidebar.mts) green across all seven layout states
at 1600/1000/393px against current master.

Co-Authored-By: Claude Fable 5 <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）',
@@ -227,6 +231,11 @@
     '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',
+    'Horizontal strip in the header, or a collapsible left sidebar (Alt+B).':
+      '会话列表显示为顶栏横向标签条，或左侧可折叠侧边栏（Alt+B）。',
     'Tall Tabs (Name + Folder)': '双行标签（名称 + 文件夹）',
     'Pop-out Button on Tabs': '标签页弹出窗口按钮',
     Panels: '面板',