fix: resolve 6 frontend-mobile build and runtime issues

- Add missing User type fields (first_name, last_name, email, avatar_url,
  city, address) used in profile.tsx
- Fix expo-constants import to use namespace import (consistent with
  rest of codebase)
- Replace placeholder production API URL with configurable value
- Fix avatar preset/color selection passing require() IDs to Image uri
  by tracking local source separately
- Replace all relative fetch() calls with configured axios instance
  (relative URLs don't resolve in React Native)

apps/frontend-mobile/types/index.ts

@@ -32,6 +32,12 @@ export interface User {
   id: number;
   username: string;
   is_admin: boolean;
+  first_name?: string;
+  last_name?: string;
+  email?: string;
+  avatar_url?: string;
+  city?: string;
+  address?: string;
 }
 
 export interface AuthResponse {