Overhaul nav, fix DB transaction bugs, add admin UI

- Replace nav user area with display name (non-clickable), gear settings
  modal, admin button (admins only), and logout button
- Settings modal handles display name, timezone, and password change
- Add admin.html + admin.js: user table with reset PW, disable/enable,
  login-as (impersonation), and delete; return-to-admin flow in nav
- Add is_admin to UserResponse so frontend can gate the Admin button
- Fix all db.begin() bugs in admin.py and users.py (transaction already
  active from get_current_user query; use commit() directly instead)
- Add email-validator and pin bcrypt==4.0.1 for passlib compatibility
- Add escHtml() to api.js and admin API namespace
- Group nav brand + links in nav-left for left-aligned layout

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

backend/requirements.txt

@@ -5,5 +5,7 @@ aiomysql==0.2.0
 pydantic-settings==2.7.0
 python-jose[cryptography]==3.3.0
 passlib[bcrypt]==1.7.4
+bcrypt==4.0.1
 python-multipart==0.0.20
 pytz==2024.2
+email-validator==2.2.0