Implement backend tree search API with frontend integration for real-time sidebar filtering with highlighted matches and vault-scoped results

frontend/style.css

@@ -804,6 +804,59 @@ select {
   display: none;
 }
 
+.filter-results-header {
+  margin-top: 8px;
+}
+
+.filter-results-group {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  padding: 4px 0 10px 12px;
+}
+
+.filter-result-item {
+  align-items: flex-start;
+  gap: 8px;
+  border-radius: 8px;
+}
+
+.filter-result-text {
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+
+.filter-result-primary,
+.filter-result-secondary {
+  min-width: 0;
+  overflow-wrap: anywhere;
+  word-break: break-word;
+}
+
+.filter-result-primary {
+  color: var(--text-primary);
+}
+
+.filter-result-secondary {
+  color: var(--text-muted);
+  font-size: 0.78rem;
+}
+
+.sidebar-filter-empty {
+  padding: 12px 16px;
+  color: var(--text-muted);
+  font-size: 0.85rem;
+}
+
+.filter-highlight {
+  background: color-mix(in srgb, var(--accent) 22%, transparent);
+  color: var(--accent);
+  padding: 0 2px;
+  border-radius: 4px;
+}
+
 /* --- Tag resize handle --- */
 .tag-resize-handle {
   height: 5px;