refactor: components read the obsidian surface from its token, not a typed rgba (3108)

Thirteen rgba(20, 23, 26, ...) across six components (the task counted
eleven). They now use --v-theme-background, which Vuetify maps to obsidian;
ArtistHeader's banner fade uses --fc-chrome-rgb, since it is nav-style
chrome. A spec fails if the literal reappears outside a comment. Values are
unchanged, so nothing should render differently.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/src/views/ExploreView.vue

@@ -369,7 +369,7 @@ onUnmounted(() => {
 
 /* Center viewer. */
 .fc-ex__viewer {
-  background: rgb(20, 23, 26);
+  background: rgb(var(--v-theme-background));
   display: flex; flex-direction: column; min-width: 0; min-height: 0;
 }
 .fc-ex__canvas { flex: 1 1 auto; display: flex; min-height: 0; min-width: 0; }