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>

README.md

@@ -683,6 +683,7 @@ Single-digit selection (1-9), color-coded status, token counts, auto-refresh. De
 | `Ctrl/Cmd+Tab`                  | Next session                                                  |
 | `Alt/Option+[` / `Alt/Option+]` | Previous / next session                                       |
 | `Alt/Option+1`-`Alt/Option+9`   | Switch to tab N (physical keys, so macOS Option layouts work) |
+| `Alt/Option+B`                  | Collapse / expand the session sidebar (sidebar layout only)   |
 | `Ctrl+Shift+{` / `Ctrl+Shift+}` | Move active tab left / right                                  |
 | `Ctrl/Cmd+C`                    | Copy selection, or interrupt when nothing is selected         |
 | `Ctrl+Shift+C`                  | Copy selection (never interrupts)                             |