feat(frontend): implement auth store and login page

Rewrites auth store with typed AuthUser shape (id, name, isAdmin) and
localStorage persistence. Adds login page with tanabata decorative
images, centered form, purple primary button matching the reference
design.

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

frontend/src/lib/stores/auth.ts

@@ -1,10 +1,15 @@
 import { writable, derived } from 'svelte/store';
-import type { User } from '$lib/api/types';
+
+export interface AuthUser {
+	id: number;
+	name: string;
+	isAdmin: boolean;
+}
 
 export interface AuthState {
 	accessToken: string | null;
 	refreshToken: string | null;
-	user: User | null;
+	user: AuthUser | null;
 }
 
 const initial: AuthState = { accessToken: null, refreshToken: null, user: null };