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/types.ts

@@ -3,6 +3,7 @@ export interface User {
   username: string;
   displayName: string;
   avatarColor: string;
+  avatar?: string | null;
   bio?: string;
   phone?: string;
   isAdmin: boolean;
@@ -15,6 +16,7 @@ export interface ChatMember {
   username: string;
   displayName: string;
   avatarColor: string;
+  avatar?: string | null;
   role: 'owner' | 'admin' | 'member';
   online: boolean;
   lastSeen: string;
@@ -27,6 +29,7 @@ export interface Chat {
   title: string;
   description?: string;
   avatarColor: string;
+  avatar?: string | null;
   isPublic?: boolean;
   role: 'owner' | 'admin' | 'member';
   lastMessage?: string;