feat: Add cloud E2E testing infrastructure

Adds Playwright tests for cloud environment with Firebase auth.

Changes:
- Refactor ComfyPage to abstract base class
- Add LocalhostComfyPage (existing devtools implementation)
- Add CloudComfyPage (cloud settings API, Firebase auth)
- Add cloud fixture with auth state persistence
- Add globalSetupCloud for Firebase login
- Add playwright.cloud.config with 5x timeout
- Add basic cloud tests (load app, canvas interaction, settings)
- Update .gitignore for auth state files
- Update tsconfig to include playwright.cloud.config

Architecture:
- ComfyPage abstract with 3 backend-specific methods
- LocalhostComfyPage uses /api/devtools + multi-user
- CloudComfyPage uses /api/settings + Firebase localStorage
- No code duplication (95% shared)

Setup:
- Requires CLOUD_TEST_EMAIL and CLOUD_TEST_PASSWORD env vars
- globalSetup logs in once, saves auth to browser_tests/.auth/
- Tests reuse saved auth state (no login per test)

playwright.cloud.config.ts

@@ -0,0 +1,34 @@
+import { defineConfig, devices } from '@playwright/test'
+
+/**
+ * Playwright configuration for cloud E2E tests.
+ * Tests run against stagingcloud.comfy.org with authenticated user.
+ */
+export default defineConfig({
+  testDir: './browser_tests/tests',
+  fullyParallel: true,
+  forbidOnly: !!process.env.CI,
+  reporter: 'html',
+
+  // Cloud tests need more time due to network latency
+  timeout: 75000, // 5x the default 15000ms
+  retries: process.env.CI ? 2 : 0,
+
+  use: {
+    trace: 'on-first-retry',
+    // Cloud URL - can override with PLAYWRIGHT_TEST_URL env var
+    baseURL: process.env.PLAYWRIGHT_TEST_URL || 'https://stagingcloud.comfy.org'
+  },
+
+  // Authenticate once before all tests
+  globalSetup: './browser_tests/globalSetupCloud.ts',
+
+  projects: [
+    {
+      name: 'chromium',
+      use: { ...devices['Desktop Chrome'] },
+      timeout: 75000,
+      grep: /@cloud/ // Only run tests tagged with @cloud
+    }
+  ]
+})