test(frontend): add page.server tests for auth flows

[fpindej]

Problem

Four public routes with meaningful server-side logic have no page.server.test.ts coverage:

• (public)/register/+page.server.ts - registration form action, captcha validation, redirect on success
• (public)/forgot-password/+page.server.ts - form action, error handling
• (public)/reset-password/+page.server.ts - token validation, redirect flows
• (public)/verify-email/+page.server.ts - token consumption, redirect on success/failure

These are critical auth paths. The existing tests for login and oauth/callback demonstrate the exact pattern to follow.

Reference

• src/frontend/src/routes/(public)/login/page.server.test.ts
• src/frontend/src/routes/(public)/oauth/callback/page.server.test.ts

Scope

One test file per route, covering:

• Happy path (successful form submission / token consumption)
• Error paths (invalid input, backend errors, expired tokens)
• Redirect behavior