feat(categories): add bidirectional piece/component category conversion

Add a "Convertir" button on piece and component category lists that allows
converting an entire category (and all its items) between piece and component.
Includes a modal with eligibility checks and blocker display.

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

app/services/modelTypes.ts

@@ -219,3 +219,33 @@ export function getModelType(id: string, opts: { signal?: AbortSignal } = {}) {
     signal: opts.signal,
   })).then(normalizeModelType);
 }
+
+export interface ConversionCheck {
+  canConvert: boolean;
+  direction: 'piece_to_component' | 'component_to_piece' | null;
+  itemCount: number;
+  names: string[];
+  blockers: string[];
+}
+
+export interface ConversionResult {
+  success: boolean;
+  convertedCount: number;
+  error?: string | null;
+}
+
+export function checkConversion(id: string, opts: { signal?: AbortSignal } = {}) {
+  const requestFetch = useRequestFetch();
+  return requestFetch<ConversionCheck>(`${ENDPOINT}/${id}/conversion-check`, createOptions({
+    method: 'GET',
+    signal: opts.signal,
+  }));
+}
+
+export function convertCategory(id: string, opts: { signal?: AbortSignal } = {}) {
+  const requestFetch = useRequestFetch();
+  return requestFetch<ConversionResult>(`${ENDPOINT}/${id}/convert`, createOptions({
+    method: 'POST',
+    signal: opts.signal,
+  }));
+}