feat(frontend): add SyncModal component

Blocking overlay polling /api/sync/status, rendering by progress.Phase
(indeterminate spinner while discovering, a real progress bar for the
activities/workouts phases), then the final sync result (success count,
or the error message that was already fetched but never rendered
before) plus the existing pending-details/pending-workouts nudge. Never
auto-closes -- not yet wired into GarminConnection.tsx (next commit).

frontend/src/App.css

@@ -485,6 +485,51 @@ button:disabled {
   word-break: break-word;
 }
 
+.sync-modal-content {
+  width: min(420px, 90vw);
+}
+
+.sync-modal-body {
+  padding: 1.5rem 1rem;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  gap: 0.75rem;
+  text-align: center;
+}
+
+.sync-modal-progress {
+  width: 100%;
+  height: 0.6rem;
+}
+
+.sync-modal-label {
+  margin: 0;
+  color: #9aa0ab;
+  font-size: 0.9rem;
+}
+
+.sync-modal-spinner {
+  width: 2rem;
+  height: 2rem;
+  border: 3px solid #2a2d35;
+  border-top-color: #3b82f6;
+  border-radius: 50%;
+  animation: sync-modal-spin 0.8s linear infinite;
+}
+
+@keyframes sync-modal-spin {
+  to {
+    transform: rotate(360deg);
+  }
+}
+
+.sync-modal-actions {
+  justify-content: flex-end;
+  padding: 0.75rem 1rem;
+  border-top: 1px solid #2a2d35;
+}
+
 .json-toggle {
   display: inline-block;
   width: 1rem;