feat: implement Phase 2 - Process Groups Management

Features added:
- Group-based process organization with collapsible cards
- Batch operations for groups (Start All, Stop All, Restart All)
- Group statistics display (running, stopped, fatal counts)
- Dedicated /groups page for group-centric management
- View toggle in /processes page (Flat view | Grouped view)

Implementation details:
- Created group API routes: /api/supervisor/groups/[name]/{start,stop,restart}
- Added React Query hooks: useStartProcessGroup, useStopProcessGroup, useRestartProcessGroup
- Created components: GroupCard, GroupView, GroupSelector
- Updated Navbar with Groups navigation link
- Integrated grouped view in processes page with toggle

Phase 2 complete (6-8 hours estimated)

components/layout/Navbar.tsx

@@ -11,6 +11,7 @@ import { cn } from '@/lib/utils/cn';
 const navItems = [
   { href: '/', label: 'Dashboard' },
   { href: '/processes', label: 'Processes' },
+  { href: '/groups', label: 'Groups' },
   { href: '/logs', label: 'Logs' },
   { href: '/config', label: 'Configuration' },
 ];