fix(fc3b): SourceFormDialog + SubscriptionsView read platforms store; drop sources.loadPlatforms

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/sources.js

@@ -7,8 +7,6 @@ export const useSourcesStore = defineStore('sources', () => {
 
   // keyed cache: null => all sources, number => artist_id filtered
   const byArtist = ref(new Map())
-  const platforms = ref([])
-  const platformsLoaded = ref(false)
   const loading = ref(false)
   const error = ref(null)
 
@@ -40,13 +38,6 @@ export const useSourcesStore = defineStore('sources', () => {
     }
   }
 
-  async function loadPlatforms() {
-    if (platformsLoaded.value) return
-    const body = await api.get('/api/sources/platforms')
-    platforms.value = body.platforms
-    platformsLoaded.value = true
-  }
-
   function _invalidate(artistId) {
     byArtist.value.delete(null)
     if (artistId != null) byArtist.value.delete(artistId)
@@ -99,9 +90,9 @@ export const useSourcesStore = defineStore('sources', () => {
   }
 
   return {
-    byArtist, platforms, platformsLoaded, loading, error,
+    byArtist, loading, error,
     allSources,
-    loadAll, loadForArtist, loadPlatforms,
+    loadAll, loadForArtist,
     create, update, remove,
     findOrCreateArtist, autocompleteArtist,
     sourcesByArtistGrouped,