fix(studio): redirect session failures to login

[iipanda]

Summary

• Redirect cookie-authenticated Studio session verification failures to /admin/login with returnTo instead of rendering the session verification error screen.
• Keep token-authentication failures inline because the host app owns bearer-token recovery.
• Document the Studio auth-gate behavior in SPEC-006 and cover the redirect helper with regression tests.

Test Plan

• bun test --cwd packages/studio ./src/lib/runtime-ui/app/admin/layout.test.tsx
• bun test --cwd packages/studio ./src
• bun run format:check
• bun run check

Note: baseline bun run unit was not used as the final gate because the clean worktree baseline hit the existing server schema.contract.test.ts database/connection failure before this change.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 66173fec-7430-468d-baa0-c59dbe69546d

📥 Commits

Reviewing files that changed from the base of the PR and between ee1789d and 325c370.

📒 Files selected for processing (3)

• docs/specs/SPEC-006-studio-runtime-and-ui.md
• packages/studio/src/lib/runtime-ui/app/admin/layout.test.tsx
• packages/studio/src/lib/runtime-ui/app/admin/layout.tsx

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/session-login-redirect

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