fix(auth): add sessionStorage fallback for web auto-login

- add shared token store backend with local/session/memory fallback
- cover fallback behavior with flutter unit tests
- add wasm e2e coverage for sessionStorage login state
- document mobile installed webapp auto-login policy

userfront-e2e/playwright.config.ts

@@ -21,13 +21,20 @@ export default defineConfig({
     screenshot: 'only-on-failure',
     video: 'retain-on-failure',
     locale: process.env.LOCALE ?? 'ko-KR',
-    serviceWorkers: 'block',
   },
   projects: [
     {
-      name: 'chromium',
+      name: 'chromium-desktop',
       use: {
         ...devices['Desktop Chrome'],
+        serviceWorkers: 'block',
+      },
+    },
+    {
+      name: 'chromium-mobile-webapp',
+      use: {
+        ...devices['Pixel 7'],
+        serviceWorkers: 'allow',
       },
     },
   ],