feat(library): serve provider entries' local art via the host art proxy

A provider plugin that runs on the host (e.g. the Playnite sync plugin) can
now reconcile cover art as an on-host FILE PATH instead of an inlined data
URL. `GET /library` rewrites such local-file art into `/library/art/<id>/
<kind>` proxy URLs (the same relative-proxy shape Steam art already uses),
and the art proxy serves the bytes from the stored path. Moonlight's
/appasset proxy reads the local file too.

This is what lets a large Playnite library sync with covers: the reconcile
payload carries paths, not bytes, so it no longer blows past the mgmt API's
request-body limit and scales to thousands of titles.

Cross-platform; local-path detection is Windows-shaped (Playnite is
Windows-only). Unit-tested (compiles + 5/5 art tests on Linux).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-host/src/library/custom.rs

@@ -92,6 +92,24 @@ pub fn load_custom() -> Vec<CustomEntry> {
     }
 }
 
+/// Serve a custom/provider entry's stored **local** art file for one [`ArtKind`] — the non-Steam
+/// branch of the art proxy (`GET /library/art/custom:<id>/<kind>`). `id` is the bare custom id (the
+/// `custom:` prefix already stripped by the handler). `None` if the entry is unknown, has no art of
+/// that kind, or that art value isn't a servable local file (e.g. an `http` URL the client fetches
+/// itself). Blocking IO — call off the async runtime.
+pub fn custom_local_art_bytes(id: &str, kind: ArtKind) -> Option<(Vec<u8>, String)> {
+    let entry = load_custom().into_iter().find(|e| e.id == id)?;
+    let field = match kind {
+        ArtKind::Portrait => entry.art.portrait,
+        ArtKind::Hero => entry.art.hero,
+        ArtKind::Logo => entry.art.logo,
+        ArtKind::Header => entry.art.header,
+    }?;
+    is_local_art_path(&field)
+        .then(|| local_art_bytes(&field))
+        .flatten()
+}
+
 fn save_custom(entries: &[CustomEntry]) -> Result<()> {
     let dir = pf_paths::config_dir();
     // Owner-private dir (0700 / SYSTEM+Admins DACL) so a non-privileged local user can't plant a