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/package.json

@@ -40,14 +40,12 @@
     "lucide-react": "^1.6.0",
     "monaco-editor": "^0.55.1",
     "motion": "^12.38.0",
-    "next-themes": "^0.4.6",
     "radix-ui": "^1.4.3",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-is": "^19.2.4",
     "react-use-measure": "^2.1.7",
     "recharts": "^3.8.1",
-    "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",
     "tailwindcss-animate": "^1.0.7",
     "xterm-addon-search": "^0.13.0",