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/app/routers/users.py

@@ -25,9 +25,8 @@ async def update_me(
     if body.timezone is not None:
         current_user.timezone = body.timezone
 
-    async with db.begin():
-        db.add(current_user)
-
+    db.add(current_user)
+    await db.commit()
     await db.refresh(current_user)
     return current_user
 
@@ -43,5 +42,5 @@ async def change_password(
 
     current_user.password_hash = hash_password(body.new_password)
 
-    async with db.begin():
-        db.add(current_user)
+    db.add(current_user)
+    await db.commit()