/signin and /signup name their Server-Timing phases and answer under 10 ms #4106

Measured on production: /signin answers app;dur=9.5 and /signup app;dur=8.8, with no phase named, for 21 KB of HTML.

Done:

• [ ] signinPage and signupPage (src/web/pages/site.ts) note session, each read they make, and render.
• [ ] A test asserts each page's phase names and an app under 10 ms in the test app.
• [ ] When a phase is the cost, the pull names it. Do not change what the pages show.

Files: src/web/pages/site.ts, a test. Before pushing: bun x tsc --noEmit and the tests you touched.