fix(tagger): provision ONNX runtime DLLs during tagger model prep

On a clean install the WD tagger download failed instantly: only the
(UI-removed) caption model prep ever downloaded the shared ONNX Runtime
DLLs, while ensure_onnx_runtime — despite its comment — only initializes
and errors when the DLL is absent. Worse, the OnceLock cached that error
for the whole session, and the onboarding step swallowed the message.

- new captioner::provision_onnx_runtime downloads missing DLLs; tagger
  prep calls it before init
- ensure_onnx_runtime no longer caches failure (Mutex<bool>, success-only
  latch) so a later download can recover in-session
- onboarding AI step now displays taggerModelError

src/components/onboarding/StepAiFeatures.tsx

@@ -8,6 +8,7 @@ export function StepAiFeatures() {
   const taggerModelStatus = useGalleryStore((s) => s.taggerModelStatus);
   const taggerModelPreparing = useGalleryStore((s) => s.taggerModelPreparing);
   const taggerModelProgress = useGalleryStore((s) => s.taggerModelProgress);
+  const taggerModelError = useGalleryStore((s) => s.taggerModelError);
   const prepareTaggerModel = useGalleryStore((s) => s.prepareTaggerModel);
   const loadTaggerModelStatus = useGalleryStore((s) => s.loadTaggerModelStatus);
 
@@ -72,6 +73,11 @@ export function StepAiFeatures() {
               ) : null}
             </div>
           ) : null}
+          {!taggerModelPreparing && taggerModelError ? (
+            <p className="mt-2 text-xs leading-relaxed text-amber-300/90">
+              Download failed: {taggerModelError}
+            </p>
+          ) : null}
         </div>
       </div>