feat(sidebar): add a rich session sidebar that carries the home screen's row detail

Session List Layout gains a third option. The old "Left sidebar" becomes
"Left sidebar simple" and is unchanged down to the byte; the new "Left sidebar"
puts on each row what the desktop home rail and the phone overview already show:
when the session was first created, how long it has been in the state it is in,
and a status pill naming that state.

A docked column is not a tab strip. It has width to spare and a row per session
either way, and "name + folder" is the whole story a TAB can tell, not the whole
story there is. This is the information that was missing, and it already existed
one surface over.

Both sidebar values are the same layout, and both set data-session-list="sidebar";
the row detail rides on a separate data-sidebar-detail attribute. That split is
the load-bearing decision here: every one of the ~25 isSessionSidebarActive()
call sites and every html[data-session-list="sidebar"] rule in styles.css and
mobile.css keeps matching both variants without being touched. A third
data-session-list value would have meant auditing and editing all of them.

- Stored values: 'header', 'sidebar' (simple), 'sidebar-rich'. Anyone already on
  'sidebar' keeps exactly the layout they picked — the rename is label-only.
- State classification and the "how long has it been like this" anchor come from
  _mobileOverviewState() / _mobileOverviewSince(), not re-derived, so the three
  surfaces cannot disagree about what "working" means. A working pane repaints
  ~1/s, so its duration is measured from the turn's last Enter: a running turn
  reads "working 12m", not "0m".
- Stamps refresh in place on a 20s clock rather than by re-rendering — a rebuild
  would restart every load spinner and alert animation in the list, twice a
  minute. The clock runs only while rich rows are on screen, and is stopped from
  both render paths and from applySessionListLayout().
- The incremental render path updates the pill, the accent class and the since
  anchor; a tick alone cannot see a state change, and a new turn re-stamps
  lastSubmitAt without changing state.
- applySessionListLayout() now re-renders on a DETAIL change too. simple <-> rich
  leaves data-session-list on 'sidebar' both times, and the meta line is emitted
  by the row template rather than toggled by CSS, so the old layout-only test
  would have flipped the setting and repainted nothing.
- Width: 300px for the extra line. The collapsed 44px rail and the handheld
  drawer are both explicitly held back from it — the desktop rule is (0,3,1) and
  would otherwise out-specify mobile.css's (0,2,1) drawer base and pin a 320px
  phone's drawer to 300px.
- Missing/stale mobile-overview.js degrades to a row with no meta line rather
  than throwing and taking the whole tab strip down.

15 new tests cover the attribute split, the solo-window override, the
detail-change re-render, the row model, both render paths, the clock lifecycle
and the mobile width guard.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/public/mobile.css

@@ -3659,6 +3659,19 @@ html[data-session-list="sidebar"] .session-sidebar {
   padding-left: var(--safe-area-left);
 }
 
+/* The rich variant's desktop column is 300px (--sidebar-width-rich, styles.css)
+   and its selector carries one attribute MORE than the drawer base above —
+   (0,3,1) vs (0,2,1) — so without this it would win here and pin the drawer of
+   a 320px phone to 300px, leaving 20px of terminal behind it. How wide a drawer
+   may be is a viewport decision, never a row-detail one: match the specificity
+   and hand the width back. Row detail itself is kept — the stamps are as useful
+   on a phone as anywhere, and the drawer is wider than the rail they were
+   designed against. */
+html[data-session-list="sidebar"][data-sidebar-detail="rich"] .session-sidebar {
+  width: min(280px, 80vw);
+  flex: 0 0 auto;
+}
+
 html[data-session-list="sidebar"] .session-sidebar.open {
   transform: translateX(0);
   visibility: visible;