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-tauri/src/tagger.rs

@@ -251,11 +251,12 @@ pub fn prepare_tagger_model_with_progress(
     std::fs::create_dir_all(&local_dir)?;
 
     // Ensure the shared ONNX runtime DLLs are present. The tagger shares
-    // them with the captioner; install them here so the tagger is fully
+    // them with the captioner; download them here so the tagger is fully
     // functional even on a clean install where the caption model has never
-    // been downloaded.
+    // been downloaded (ensure_onnx_runtime only initializes, never downloads).
     let caption_model_dir = crate::captioner::model_dir(app_data_dir);
     std::fs::create_dir_all(&caption_model_dir)?;
+    crate::captioner::provision_onnx_runtime(&caption_model_dir)?;
     crate::captioner::ensure_onnx_runtime(&caption_model_dir)?;
 
     // Download the two tagger-specific files.