frontend: add LoginGate, wire session into App, add logout link

frontend/src/App.css

@@ -57,7 +57,6 @@ body {
    alongside Activities/Progression/Training plan, so it reads more like an
    account chip (à la Slack/GitHub's corner avatar) than another nav tab. */
 .profile-name {
-  margin-left: auto;
   display: inline-flex;
   align-items: center;
   gap: 0.4rem;
@@ -87,6 +86,23 @@ body {
   color: white;
 }
 
+.header-actions {
+  margin-left: auto;
+  display: flex;
+  align-items: center;
+  gap: 0.6rem;
+}
+
+.logout-link {
+  color: #9aa0ab;
+  font-size: 0.85rem;
+  text-decoration: none;
+}
+
+.logout-link:hover {
+  color: #3b82f6;
+}
+
 .garmin-connection {
   display: flex;
   flex-direction: column;