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/subagent-windows.js

@@ -401,15 +401,12 @@ Object.assign(CodemanApp.prototype, {
           continue;
         }
 
-        // Draw curved line from TAB bottom-center to window top-center
-        const x1 = tabRect.left + tabRect.width / 2;
-        const y1 = tabRect.bottom;
-        const x2 = winRect.left + winRect.width / 2;
-        const y2 = winRect.top;
-
-        // Bezier curve control points for smooth curve
-        const midY = (y1 + y2) / 2;
-        const path = `M ${x1} ${y1} C ${x1} ${midY}, ${x2} ${midY}, ${x2} ${y2}`;
+        // Draw a curved line from the tab to the window. Header strip: tab
+        // bottom-center → window top-center (vertical). Sidebar: tab right-edge →
+        // window left-edge (horizontal), otherwise the curve loops backwards
+        // underneath the sidebar. _tabAnchor/_tabConnectorPath live in app.js.
+        const anchor = this._tabAnchor(tabRect);
+        const path = this._tabConnectorPath(anchor, winRect);
 
         const line = document.createElementNS('http://www.w3.org/2000/svg', 'path');
         line.setAttribute('d', path);
@@ -744,9 +741,11 @@ Object.assign(CodemanApp.prototype, {
       win.style.top = `${finalY}px`;
       win.style.bottom = 'auto';
     } else if (flyFromTab) {
-      const tabRect = parentTab.getBoundingClientRect();
-      win.style.left = `${tabRect.left}px`;
-      win.style.top = `${tabRect.bottom}px`;
+      // Spawn at the tab: below it in header layout, to its RIGHT in sidebar
+      // layout — spawning at tabRect.left there would land on top of the sidebar.
+      const anchor = this._tabAnchor(parentTab.getBoundingClientRect());
+      win.style.left = `${anchor.spawnLeft}px`;
+      win.style.top = `${anchor.spawnTop}px`;
       win.style.transform = 'scale(0.3)';
       win.style.opacity = '0';
       win.classList.add('spawning');
@@ -1221,6 +1220,19 @@ Object.assign(CodemanApp.prototype, {
     dropdown.style.left = `${rect.left + rect.width / 2}px`;
     dropdown.style.transform = 'translateX(-50%)';
     dropdown.classList.add('open');
+
+    // Keep it on screen. A badge in the left sidebar — and above all one in the
+    // 44px collapsed rail — sits so far left that a centre-anchored dropdown
+    // hangs off the viewport. Measured after .open so it has a box; a no-op
+    // whenever the centred position already fits, so header layout is unchanged.
+    const dropRect = dropdown.getBoundingClientRect();
+    const overflowLeft = 8 - dropRect.left;
+    const overflowRight = dropRect.right - (window.innerWidth - 8);
+    if (overflowLeft > 0) {
+      dropdown.style.transform = `translateX(calc(-50% + ${Math.round(overflowLeft)}px))`;
+    } else if (overflowRight > 0) {
+      dropdown.style.transform = `translateX(calc(-50% - ${Math.round(overflowRight)}px))`;
+    }
   },
 
   // Schedule hide after delay (allows moving mouse to dropdown)