feat(pinchflat): Media Profiles — named download presets per Library source

A MediaProfile bundles { kind, quality, options, outputTemplate, extraArgs }
that a Source points at via Source.profileId, overriding the global defaults
field-by-field for that source's downloads.

Backend mirrors CommandTemplate: profiles.ts (createJsonStore, PROFILES_MAX,
isProfileLike + sanitize), CRUD IPC (profiles:list/save/remove) + preload +
useProfiles store; Source.profileId + sources:set-profile. Pure resolution
in @shared/profileResolve.ts (profile → global, per field; unit-tested).
Applied in the sources-store enqueueItems; a per-download collectionOutput-
Template override threads to buildArgs so a profile's outputTemplate wins.

UI: ProfileManager (mirrors TemplateManager) in a new Settings → Advanced →
Media profiles card, plus a per-source Profile picker in the Library source
detail. Both live-verified in the preview; 354 tests pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/sources.ts

@@ -51,6 +51,21 @@ export function setSourceWatched(id: string, watched: boolean): Source[] {
   return sourcesStore.write(listSources().map((s) => (s.id === id ? { ...s, watched } : s)))
 }
 
+/**
+ * Point a source at a MediaProfile, or clear it (profileId === null). Returns
+ * all sources. A cleared profile drops the field entirely so the record stays
+ * clean rather than carrying an explicit undefined.
+ */
+export function setSourceProfile(id: string, profileId: string | null): Source[] {
+  return sourcesStore.write(
+    listSources().map((s) => {
+      if (s.id !== id) return s
+      const { profileId: _drop, ...rest } = s
+      return profileId ? { ...rest, profileId } : rest
+    })
+  )
+}
+
 /** Remove a source and all of its media items. Returns the remaining sources. */
 export function removeSource(id: string): Source[] {
   const sources = sourcesStore.write(listSources().filter((s) => s.id !== id))