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/schemas.ts

@@ -945,6 +945,8 @@ export const SettingsUpdateSchema = z
     // CODEMAN_ALLOW_UNAUTHENTICATED_NETWORK env var. Stripped before persisting.
     acknowledgeUnauthTunnel: z.boolean().optional(),
     tabTwoRows: z.boolean().optional(),
+    /** Session list layout: 'header' = horizontal tab strip, 'sidebar' = collapsible left sidebar. Display key (per-device). */
+    sessionListLayout: z.enum(['header', 'sidebar']).optional(),
     agentTeamsEnabled: z.boolean().optional(),
     /** Model for new Claude sessions (e.g. "claude-fable-5[1m]", "opus[1m]"); takes precedence over opusContext1mEnabled */
     claudeModel: z.string().max(50).optional(),