feat(tagger): add JoyTag provider behind a tagger-model abstraction

Introduces a selectable tagging model so Phokus isn't locked to the
anime-leaning WD tagger. JoyTag uses the Danbooru schema but generalizes to
photographic content and is strong on NSFW concepts — a better fit for a photo
manager while keeping the explicitness range.

- `TaggerModel` enum (wd | joytag) persisted as a `tagger_model` setting;
  `model_dir`, status, and download now follow the active model (per-model
  repo/dir/file list). WD stays the default.
- `Tagger` trait implemented by `WdTagger` and the new `JoyTagger`;
  `create_active_tagger` builds the selected one. The worker holds
  `Box<dyn Tagger>` and rebuilds on model change (TAGGER_SESSION_DIRTY).
- Shared `assemble_batch` skeleton (pack -> one forward pass -> per-image
  fallback, results in input order); both providers and the shared
  decode/pad/resize are de-duped onto common helpers.
- JoyTag specifics: NCHW + RGB + CLIP-normalized input (vs WD's NHWC/BGR/raw),
  flat top_tags.txt labels, logits -> sigmoid -> threshold (default 0.4). It has
  no native rating, so the explicitness rating is derived from its NSFW tags.
- Tags are attributed to the model that produced them (ai_tagger_model),
  via Tagger::model_name, instead of a hardcoded WD constant.
- New get/set_tagger_model commands.

Backend only; the Settings model picker and end-to-end testing against the
JoyTag model files come next.

src-tauri/src/lib.rs

@@ -202,6 +202,8 @@ pub fn run() {
             commands::get_tagger_model_status,
             commands::get_tagger_acceleration,
             commands::set_tagger_acceleration,
+            commands::get_tagger_model,
+            commands::set_tagger_model,
             commands::probe_tagger_runtime,
             commands::get_tagger_threshold,
             commands::set_tagger_threshold,