Usage panel: scope to open panes, lead with tokens, label $ as API estimate

Addresses feedback on the usage panel:
- It was summing every recent session on the distro (all projects, mounted
  + home dirs), not the open panes' work — which read as inflated/double-
  counted. (Verified there's no literal double count: every transcript is
  read once and no two project dirs share a cwd, since claude resolves
  symlinks/mounts to the real path before mangling.) Now the panel + the
  titlebar chip default to sessions whose cwd matches an open pane, with an
  'open panes / all recent' toggle to see the full per-distro list.
- Token volume is now the headline figure; the API-cost estimate is shown
  as a clearly-labeled '~$' secondary, with a footer note that it's n/a on
  a Pro/Max subscription and can't reflect /usage quota. Kept visible (not
  hidden) so it can be validated against real API billing.

Frontend-only; backend still returns the full recent set for the toggle.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -804,6 +804,13 @@ export default function App() {
     [tree],
   );
 
+  // Titlebar chip total — scoped to the open panes ("this workspace"), matching
+  // the usage panel's default view, so it isn't inflated by unrelated projects.
+  const workspaceUsageTotal = useMemo(() => {
+    const cwds = new Set(openPanes.map((p) => p.cwd).filter(Boolean));
+    return totalCost(usageSessions.filter((s) => cwds.has(s.cwd)));
+  }, [openPanes, usageSessions]);
+
   // Outside-click dismissal for the titlebar dropdowns. Mirrors the
   // per-pane shell-picker pattern in LeafPane.tsx.
   useEffect(() => {
@@ -2153,7 +2160,7 @@ export default function App() {
           title="claude token usage & estimated cost (Ctrl+Shift+U)"
           aria-label="Usage"
         >
-          💰{usageSessions.length > 0 ? ` ${formatUsd(totalCost(usageSessions))}` : ""}
+          💰{workspaceUsageTotal > 0 ? ` ~${formatUsd(workspaceUsageTotal)}` : ""}
         </button>
         <button
           className="palette-btn"