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/ui/toast-store.ts

@@ -0,0 +1,51 @@
+import { useSyncExternalStore } from 'react';
+
+export type ToastType = 'success' | 'error' | 'warning' | 'info';
+
+export interface Toast {
+  id: string;
+  type: ToastType;
+  message: string;
+  createdAt: number;
+}
+
+let toasts: Toast[] = [];
+const listeners: Set<() => void> = new Set();
+let idCounter = 0;
+
+function notify() {
+  listeners.forEach((fn) => fn());
+}
+
+function addToast(type: ToastType, message: string) {
+  const id = `toast-${++idCounter}-${Date.now()}`;
+  toasts = [...toasts, { id, type, message, createdAt: Date.now() }];
+  notify();
+}
+
+export function removeToast(id: string) {
+  toasts = toasts.filter((t) => t.id !== id);
+  notify();
+}
+
+function subscribe(callback: () => void) {
+  listeners.add(callback);
+  return () => {
+    listeners.delete(callback);
+  };
+}
+
+function getSnapshot() {
+  return toasts;
+}
+
+export function useToasts() {
+  return useSyncExternalStore(subscribe, getSnapshot);
+}
+
+export const toast = {
+  success: (message: string) => addToast('success', message),
+  error: (message: string) => addToast('error', message),
+  warning: (message: string) => addToast('warning', message),
+  info: (message: string) => addToast('info', message),
+};