fix(favorites): correct post-merge issues from PR #184

- btn-sm: add global .btn-sm modifier to theme.css (was only scoped inside device-sync)
- year filter: exclude songs without year metadata when filter is active
- showing count: only render "X of Y" label when a filter is actually active

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/styles/theme.css

@@ -3711,6 +3711,12 @@ body.psy-dragging * {
   transform: translateY(0);
 }
 
+.btn-sm {
+  padding: 3px 8px;
+  font-size: 12px;
+  gap: var(--space-1);
+}
+
 /* ─── Input ─── */
 .input {
   width: 100%;