Replace Sonner with custom Sera UI-inspired toast system (#296)

* refactor(ui): replace Sonner with custom Sera UI-inspired toast system

Replace the Sonner toast library with a custom implementation inspired by
Sera UI's glassmorphism design. The new system uses an external store pattern
with useSyncExternalStore for React integration, keeping the same
toast.success()/error()/warning() API so all 172 call sites required only
an import path change.

Key changes:
- New toast-store.ts: singleton store with identical API to Sonner
- New toast.tsx: Sera UI-faithful Notification component with Framer Motion
  animations, frosted glass (backdrop-blur-xl), type gradient overlays,
  animated progress bar (green→blue→sky gradient), and hover:scale-105
- Removed sonner and next-themes dependencies
- Rewired all 19 consumer files to import from the new store

* fix(ui): resolve ESLint errors in toast system

- Use const for listeners Set (prefer-const)
- Initialize startRef with 0 instead of Date.now() to satisfy
  react-hooks/purity rule, set actual value inside useEffect

frontend/src/components/StackAlertSheet.tsx

@@ -12,7 +12,7 @@ import { Label } from '@/components/ui/label';
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '@/components/ui/select';
 import { Tooltip, TooltipContent, TooltipProvider, TooltipTrigger } from '@/components/ui/tooltip';
 import { Trash2, HelpCircle, AlertTriangle, Info, CheckCircle2, Loader2 } from 'lucide-react';
-import { toast } from 'sonner';
+import { toast } from '@/components/ui/toast-store';
 import { apiFetch } from '@/lib/api';
 import { useNodes } from '@/context/NodeContext';
 import { useAuth } from '@/context/AuthContext';