Add user management to admin dashboard and refine button specificity

- Implement admin user management interface (, , ) with async CRUD and password updating
- Add Users tab link to admin navigation across  and
- Use  in  to lower specificity for native button resets, allowing modifier classes like  to take precedence

styles.css

@@ -470,7 +470,10 @@
     transform: translateY(-1px);
   }
 
-  button.btn {
+  /* :where() keeps this native-button reset at zero specificity, so
+     modifier classes like .btn-primary's background can actually win
+     instead of losing to button.btn's higher (0,1,1) specificity */
+  :where(button.btn) {
     cursor: pointer;
     background: transparent;
   }