Collapse sidebar nav groups into dropdowns that open on the active page

Each group label (Dashboard, Settings) toggles its items; a group starts
closed unless it holds the current route, and a manual toggle resets on
the next navigation. Mobile keeps every item visible.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/components/Icon.tsx

@@ -17,6 +17,7 @@ const paths: Record<string, string> = {
   search: '<circle cx="10" cy="10" r="6"/><path d="m15 15 5 5"/>',
   info: '<circle cx="12" cy="12" r="9"/><path d="M12 11v6M12 7v.2"/>',
   check: '<path d="m5 12 4 4L19 6"/>',
+  'chevron-down': '<path d="m6 9 6 6 6-6"/>',
   'chevron-updown': '<path d="m8 9 4-4 4 4M8 15l4 4 4-4"/>',
   logout:'<path d="M10 4H4v16h6M10 12h11m-4-4 4 4-4 4"/>',
   plus: '<path d="M12 4v16M4 12h16"/>',