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/mobile-handlers.js

@@ -168,6 +168,11 @@ const MobileDetection = {
       resizeTimeout = setTimeout(() => {
         this.updateBodyClass();
         this.updateAppHeight();
+        // Whether the session sidebar is a docked column or a modal overlay is
+        // decided at 1024px, so crossing that width has to re-sync the drawer
+        // state — otherwise the `inert`/aria-hidden set on a closed overlay
+        // drawer survives into the docked rail and makes it unclickable.
+        if (typeof app !== 'undefined') app.applySessionListLayout?.();
         // Tab auto-wrap is width-driven, so it must re-evaluate on resize — the only
         // other trigger is a tab content render. No-op on mobile/tablet (method bails).
         if (typeof app !== 'undefined') app.updateTabOverflowMode?.();
@@ -581,6 +586,7 @@ const SwipeHandler = {
   _touchStartHandler: null,
   _touchEndHandler: null,
   _element: null,
+  _ignoreGesture: false,
 
   /** Initialize swipe handling */
   init() {
@@ -609,6 +615,12 @@ const SwipeHandler = {
   },
 
   onTouchStart(e) {
+    // The session sidebar is an overlay child of .main, so its touches bubble in
+    // here. Swiping across the open session drawer — the natural "dismiss it"
+    // gesture — would otherwise fire nextSession() and drop the user into a
+    // session they never tapped.
+    this._ignoreGesture = !!e.target?.closest?.('.session-sidebar');
+    if (this._ignoreGesture) return;
     if (!e.touches || e.touches.length !== 1) return;
     this.startX = e.touches[0].clientX;
     this.startY = e.touches[0].clientY;
@@ -616,6 +628,10 @@ const SwipeHandler = {
   },
 
   onTouchEnd(e) {
+    if (this._ignoreGesture) {
+      this._ignoreGesture = false;
+      return;
+    }
     if (!e.changedTouches || e.changedTouches.length !== 1) return;
 
     const endX = e.changedTouches[0].clientX;