feat: avatar upload for profile/chats, fix session persistence on refresh

- Profile modal: upload/remove user avatar, edit name/bio/phone, change password
- Chat info panel: upload/remove chat photo (owner/admin only)
- Avatar component: render photo when available, fallback to initials
- Session fix: initialize Zustand store from localStorage to prevent redirect on refresh
- Backend: avatar upload endpoints for users and chats, migration adds avatar columns

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/store/index.ts

@@ -26,8 +26,15 @@ interface AppStore {
   logout: () => void;
 }
 
+function loadUser(): User | null {
+  try {
+    const s = localStorage.getItem('jc_user');
+    return s ? JSON.parse(s) : null;
+  } catch { return null; }
+}
+
 export const useStore = create<AppStore>((set, get) => ({
-  user: null,
+  user: loadUser(),
   chats: [],
   activeChat: null,
   messages: {},