Remove CSRF, add password change, admin user management

Stripped all CSRF token generation, injection, and validation since it
breaks behind Apache reverse proxy. Removed handlers/csrf.go, stripped
CSRFToken from PageData, removed validateCSRFToken from all POST handlers,
and cleaned up hidden inputs and JS CSRF references.

Added self-service password change at /password/ with current-password
verification and bcrypt update. New Password link in navbar.

Extended admin panel with user management: lists all users with join dates
and allows admins to delete other users (self-deletion blocked). Added
GetAllUsers() and DeleteUser() to models.

templates/pages/thread.html

@@ -105,11 +105,3 @@
 </body>
 </html>
 {{end}}
-{ target.classList.remove('post-highlighted'); }, 2000);
-            }
-        });
-    });
-    </script>
-</body>
-</html>
-{{end}}