UI: lists in grouped surface cards; debug demo seeder

- Per-account lists now render inside rounded surfaceContainer cards
  (grouped-list pattern) with inset dividers and transparent rows, instead
  of bare ListItems on the background — surfaces-first per design feedback.
- Add debug-only DemoSeeder (local, non-syncing 'Floret Demo' list) behind
  BuildConfig.DEBUG + an intent extra, to populate the UI for design review.

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

app/build.gradle.kts

@@ -70,6 +70,7 @@ android {
 
     buildFeatures {
         compose = true
+        buildConfig = true
     }
 
     packaging {