feat: colored count badge on Colors sidebar entry

Adds a `colored` field to /library/stats counting non-discarded
photos with a color_label set, mirroring how `rated` is exposed.
The Colors sidebar entry now shows the live count and refreshes
through the existing LIBRARY_STATS_QUERY_KEY invalidations.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -409,6 +409,7 @@ export interface DuplicateGroupsResponse {
 export interface LibraryStats {
   all_photos: number
   rated: number
+  colored: number
   duplicates: number
   discarded: number
   total_photos: number