perf(manager-dashboard): paginate users instead of loading entire collection

[roiguri]

Refs #267

Source: Deep code review — docs/code-review-2026-05.md (finding P3, P2).

Where: src/app/pages/manager-dashboard-page.js:184

Issue: Loads the entire users collection; read cost grows quadratically as users grow.

Fix: Paginate (limit(50) + cursor); add server-side search by email/name (where('email','>=',...)). If a full export is genuinely needed, do it via a Cloud Function with the admin SDK. Best built on the shared pagination utility (R4).