Let the user choose which SCRFD finds their faces

faces.md §12.3 measured what the cheapest detector costs: the small
faces in every group shot, and a dog embedded a dozen times. Which
trade is right depends on the machine doing the sweep — a desktop left
overnight and a tablet on a battery want different answers — so the
detector is now a per-device setting, Fast / Balanced / Thorough on
the settings page beside the indexing button, persisted with the rest
of the settings file.

A detector is half of a model id. Every face, marker, shard and
calibration is keyed on faces.model_id precisely so that a model change
is a new id and a re-index rather than a silent change under existing
data, and a detector change is a model change: it decides which faces
exist and where the landmarks that align them land. So each choice
names its own pipeline. 500M keeps the bare "w600k_mbf" every existing
library was written under, so an upgrade disturbs nothing; the others
are qualified. Choosing one restarts coverage from zero under the new
id, the sweep re-detects, confirmed names carry across by box overlap,
and the sync shards are keyed by the same id so a peer on another
setting neither adopts nor pollutes them. The library controller
carries the id into the sync the same way it carries the cache budget,
because the sync starts from places that have no settings in reach.

All three shape-fixed exports ship — APK, Arch, Flatpak — since a
tablet has no other way to obtain the one it was not installed with;
the APK grows by twenty megabytes for the choice.

ui/dr-ui/src/library.rs

@@ -4419,12 +4419,15 @@ pub fn shared_face_models_dir() -> PathBuf {
 /// 3. **The system directories.** Where a package installs them — the Arch
 ///    package puts the pair in `/usr/share/darkroom/models`. Last, so anything
 ///    the user placed themselves outranks what the package shipped.
-pub fn face_models(account: &Account) -> Option<(PathBuf, PathBuf)> {
-    fn pair(dir: PathBuf) -> Option<(PathBuf, PathBuf)> {
-        let detector = dir.join("scrfd_500m_640.onnx");
+pub fn face_models(
+    account: &Account,
+    detector: dr_types::FaceDetector,
+) -> Option<(PathBuf, PathBuf)> {
+    let pair = |dir: PathBuf| {
+        let detector = dir.join(detector.file_name());
         let embedder = dir.join("arcface_mbf_b1.onnx");
         (detector.is_file() && embedder.is_file()).then_some((detector, embedder))
-    }
+    };
     pair(face_models_dir(account))
         .or_else(|| pair(shared_face_models_dir()))
         .or_else(|| system_face_models_dirs().into_iter().find_map(pair))