Add user password change functionality

Introduced a backend API endpoint for changing user passwords with validation. Added a new "User Settings" page in the frontend to allow users to update their passwords, including a matching UI update for navigation and styles.

frontend/src/styles.css

@@ -1115,6 +1115,39 @@ button {
   border-color: #38bdf8;
 }
 
+.profile-btn {
+  width: 100%;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  margin-top: 8px;
+  border: 1px solid #3a63a1;
+  border-radius: 10px;
+  background: linear-gradient(180deg, #15315d, #11274c);
+  color: #e7f2ff;
+  min-height: 40px;
+  font-weight: 650;
+}
+
+.profile-btn:hover {
+  border-color: #58b0e8;
+  background: linear-gradient(180deg, #1a427a, #15335f);
+}
+
+.profile-btn.active {
+  border-color: #66c7f4;
+  box-shadow: inset 0 0 0 1px #66c7f455;
+}
+
+.user-settings-page h2 {
+  margin-top: 4px;
+  margin-bottom: 4px;
+}
+
+.user-settings-card {
+  max-width: 760px;
+}
+
 table {
   width: 100%;
   border-collapse: collapse;