chore: onboarding/MFA-modal WIP, dev-workflow guidance, ideas updates

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/App.css

@@ -211,6 +211,11 @@ input[type="color"] {
   cursor: pointer;
 }
 
+.field-hint {
+  font-size: 0.75rem;
+  color: #6b7280;
+}
+
 button:hover:not(:disabled) {
   border-color: #3b82f6;
 }
@@ -526,6 +531,29 @@ button:disabled {
   border-top: 1px solid #2a2d35;
 }
 
+.garmin-mfa-modal-content {
+  width: min(420px, 90vw);
+}
+
+.garmin-mfa-modal-body {
+  padding: 1.5rem 1rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.75rem;
+}
+
+.garmin-mfa-modal-message {
+  margin: 0;
+  color: #9aa0ab;
+  font-size: 0.9rem;
+}
+
+.garmin-mfa-modal-actions {
+  justify-content: flex-end;
+  padding: 0.75rem 1rem;
+  border-top: 1px solid #2a2d35;
+}
+
 .json-toggle {
   display: inline-block;
   width: 1rem;