feat: add auto-archive timeout setting

[webwww123]

Summary

Adds a minimal web-only auto-archive timeout for active sessions.

What changed

• adds an Auto-Archive Timeout setting in the web Settings page
• stores the timeout in localStorage
• automatically archives active sessions after they stay idle past the selected timeout
• skips auto-archiving while a session is still thinking or waiting on permission requests
• adds settings page tests for the new preference

Implementation notes

This keeps the change intentionally small:

• no protocol changes
• no hub schema changes
• no production deployment behavior changes unless a user enables the setting in the web UI

Idle detection uses the existing session summary state and the current archive API.

Closes #278

Validation

docker run --rm -v "/root/sandbox/hapi-pr:/workspace:ro" oven/bun:1 sh -lc "cp -a /workspace /tmp/hapi-pr && cd /tmp/hapi-pr/web && bun install && bunx vitest run src/routes/settings/index.test.tsx && bun run typecheck"