Add multi-user auth, admin panel, and timezone support; rename to Yolkbook

- Rename app from Eggtracker to Yolkbook throughout
- Add JWT-based authentication (python-jose, passlib/bcrypt)
- Add users table; all data tables gain user_id FK for full data isolation
- Super admin credentials sourced from ADMIN_USERNAME/ADMIN_PASSWORD env vars,
  synced on every startup; orphaned rows auto-assigned to admin post-migration
- Login page with self-registration; JWT stored in localStorage (30-day expiry)
- Admin panel (/admin): list users, reset passwords, disable/enable, delete,
  and impersonate (Login As) with Return to Admin banner
- Settings modal (gear icon in nav): timezone selector and change password
- Timezone stored per-user; stats date windows computed in user's timezone;
  date input setToday() respects user timezone via Intl API
- migrate_v2.sql for existing single-user installs
- Auto-migration adds timezone column to users on startup
- Updated README with full setup, auth, admin, and migration docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nginx/html/js/summary.js

@@ -187,7 +187,7 @@ async function exportCSV() {
         a.href     = url;
         const now      = new Date();
         const fileDate = `${now.getFullYear()}-${String(now.getMonth()+1).padStart(2,'0')}-${String(now.getDate()).padStart(2,'0')}`;
-        a.download = `egg-tracker-${fileDate}.csv`;
+        a.download = `yolkbook-${fileDate}.csv`;
         a.click();
         URL.revokeObjectURL(url);