feat(frontend): add dev mock API plugin

Adds a Vite dev-only middleware that intercepts /api/v1/* requests
and returns mock responses for auth, users, files, tags, categories,
and pools. Login with any username and password "password".

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

frontend/vite.config.ts

@@ -1,7 +1,8 @@
 import { sveltekit } from '@sveltejs/kit/vite';
 import tailwindcss from '@tailwindcss/vite';
 import { defineConfig } from 'vite';
+import { mockApiPlugin } from './vite-mock-plugin';
 
 export default defineConfig({
-	plugins: [tailwindcss(), sveltekit()]
+	plugins: [tailwindcss(), sveltekit(), mockApiPlugin()]
 });