feat(web): add unread session indicators

[webwww123]

Summary

• add a local read/unread state for sessions in the web app
• mark a session as unread when it has newer updates than the last recorded read timestamp
• clear the unread state when the session is opened
• keep groups with unread sessions expanded by default so the new activity is visible

Why

Issue #270 asks for a simple way to notice when another session has finished or produced new output.

This PR keeps the implementation intentionally small and web-only:

• read state is stored in localStorage
• no hub / protocol changes
• no production deployment changes

Validation

• docker run --rm -v /root/sandbox/hapi-pr:/workspace -w /workspace oven/bun:1 sh -lc "bun install && cd web && bunx vitest run src/lib/sessionReadState.test.ts && bun run typecheck"