test: add authenticated upload e2e flow

[hhhjin]

Summary

• add a Playwright-authenticated upload flow with shared auth bootstrap helpers
• add test-only auth routes for seeding an authenticated Better Auth session in E2E
• update preview/proxy setup so production-style E2E runs can cover authenticated and anonymous upload paths

Testing

• pnpm test:e2e -- --project=chromium e2e/authenticated-upload.spec.ts
• pnpm test:e2e -- --project=chromium e2e/public-upload.spec.ts

[gemini-code-assist]

Code Review

This pull request introduces an end-to-end (E2E) testing framework using Playwright, specifically focusing on authenticated upload flows. It adds a dedicated test authentication router to simulate user sessions, updates the frontend to provide a readiness signal for tests, and configures Playwright to handle authenticated states. The review feedback identifies a critical issue in the test authentication route where Hono's signed cookie utilities are used for Better Auth session tokens; since Better Auth expects raw tokens, the signatures appended by Hono would cause authentication to fail.