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>

.changeset/9008093b.md

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+"aicodeman": minor
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+Add an optional collapsible left session sidebar as an alternative to the header tab strip.
+
+With many concurrent sessions the horizontal strip wraps into several rows and stops being scannable. The new layout puts the session list in a vertical `<aside>` with a filter box and a live session count, collapsible to a 44px rail that keeps the status dots and task badges visible.
+
+Opt-in via Settings → Display → Tab Bar → Session List Layout; the default stays the header strip, so nothing changes unless you switch. Both layouts share one `#sessionTabs` element that is re-parented between mount points, so every existing affordance (status, mode badge, alerts, drag-reorder, keyboard navigation, web tabs, subagent windows) behaves identically in both. Below 1024px the sidebar is an off-canvas drawer that overlays the terminal instead of shrinking it. Collapse state persists per device; `Alt+B` toggles it.