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/groups/GroupSelector.tsx

@@ -0,0 +1,37 @@
+'use client';
+
+import { Button } from '@/components/ui/button';
+import { LayoutGrid, List } from 'lucide-react';
+
+interface GroupSelectorProps {
+  viewMode: 'flat' | 'grouped';
+  onViewModeChange: (mode: 'flat' | 'grouped') => void;
+}
+
+export function GroupSelector({ viewMode, onViewModeChange }: GroupSelectorProps) {
+  return (
+    <div className="flex gap-2 items-center">
+      <span className="text-sm font-medium text-muted-foreground">View:</span>
+      <div className="flex gap-1 border rounded-md p-1">
+        <Button
+          variant={viewMode === 'flat' ? 'default' : 'ghost'}
+          size="sm"
+          onClick={() => onViewModeChange('flat')}
+          className="gap-2"
+        >
+          <LayoutGrid className="h-4 w-4" />
+          Flat
+        </Button>
+        <Button
+          variant={viewMode === 'grouped' ? 'default' : 'ghost'}
+          size="sm"
+          onClick={() => onViewModeChange('grouped')}
+          className="gap-2"
+        >
+          <List className="h-4 w-4" />
+          Grouped
+        </Button>
+      </div>
+    </div>
+  );
+}