diff --git a/frontend/vite-mock-plugin.ts b/frontend/vite-mock-plugin.ts
new file mode 100644
index 0000000..f9ad4fa
--- /dev/null
+++ b/frontend/vite-mock-plugin.ts
@@ -0,0 +1,133 @@
+/**
+ * Dev-only Vite plugin that intercepts /api/v1/* and returns mock responses.
+ * Login: any username + password "password" succeeds.
+ */
+import type { Plugin } from 'vite';
+import type { IncomingMessage, ServerResponse } from 'http';
+
+function readBody(req: IncomingMessage): Promise<unknown> {
+	return new Promise((resolve) => {
+		let data = '';
+		req.on('data', (chunk) => (data += chunk));
+		req.on('end', () => {
+			try {
+				resolve(data ? JSON.parse(data) : {});
+			} catch {
+				resolve({});
+			}
+		});
+	});
+}
+
+function json(res: ServerResponse, status: number, body: unknown) {
+	const payload = JSON.stringify(body);
+	res.writeHead(status, {
+		'Content-Type': 'application/json',
+		'Content-Length': Buffer.byteLength(payload),
+	});
+	res.end(payload);
+}
+
+function noContent(res: ServerResponse) {
+	res.writeHead(204);
+	res.end();
+}
+
+const MOCK_ACCESS_TOKEN = 'mock-access-token';
+const MOCK_REFRESH_TOKEN = 'mock-refresh-token';
+
+const TOKEN_PAIR = {
+	access_token: MOCK_ACCESS_TOKEN,
+	refresh_token: MOCK_REFRESH_TOKEN,
+	expires_in: 900,
+};
+
+const ME = {
+	id: 1,
+	name: 'admin',
+	is_admin: true,
+	can_create: true,
+	is_blocked: false,
+};
+
+export function mockApiPlugin(): Plugin {
+	return {
+		name: 'mock-api',
+		configureServer(server) {
+			server.middlewares.use(async (req, res, next) => {
+				const url = req.url ?? '';
+				const method = req.method ?? 'GET';
+
+				if (!url.startsWith('/api/v1')) {
+					return next();
+				}
+
+				const path = url.replace('/api/v1', '').split('?')[0];
+
+				// POST /auth/login
+				if (method === 'POST' && path === '/auth/login') {
+					const body = (await readBody(req)) as Record<string, string>;
+					if (body.password === 'password') {
+						return json(res, 200, TOKEN_PAIR);
+					}
+					return json(res, 401, { code: 'unauthorized', message: 'Invalid credentials' });
+				}
+
+				// POST /auth/refresh
+				if (method === 'POST' && path === '/auth/refresh') {
+					return json(res, 200, TOKEN_PAIR);
+				}
+
+				// POST /auth/logout
+				if (method === 'POST' && path === '/auth/logout') {
+					return noContent(res);
+				}
+
+				// GET /auth/sessions
+				if (method === 'GET' && path === '/auth/sessions') {
+					return json(res, 200, {
+						items: [
+							{
+								id: 1,
+								user_agent: 'Mock Browser',
+								started_at: new Date().toISOString(),
+								expires_at: null,
+								last_activity: new Date().toISOString(),
+								is_current: true,
+							},
+						],
+						total: 1,
+					});
+				}
+
+				// GET /users/me
+				if (method === 'GET' && path === '/users/me') {
+					return json(res, 200, ME);
+				}
+
+				// GET /files
+				if (method === 'GET' && path === '/files') {
+					return json(res, 200, { items: [], next_cursor: null, prev_cursor: null });
+				}
+
+				// GET /tags
+				if (method === 'GET' && path === '/tags') {
+					return json(res, 200, { items: [], total: 0, offset: 0, limit: 50 });
+				}
+
+				// GET /categories
+				if (method === 'GET' && path === '/categories') {
+					return json(res, 200, { items: [], total: 0, offset: 0, limit: 50 });
+				}
+
+				// GET /pools
+				if (method === 'GET' && path === '/pools') {
+					return json(res, 200, { items: [], total: 0, offset: 0, limit: 50 });
+				}
+
+				// Fallback: 404
+				return json(res, 404, { code: 'not_found', message: `Mock: no handler for ${method} ${path}` });
+			});
+		},
+	};
+}
\ No newline at end of file
diff --git a/frontend/vite.config.ts b/frontend/vite.config.ts
index bf699a8..fa89ef0 100644
--- a/frontend/vite.config.ts
+++ b/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()]
 });