Extract season overview/workflow cards and add sponsor seed data

Splits AdminSeasonsView into AdminSeasonOverviewCard and AdminSeasonWorkflowCard
for cleaner separation of concerns. Adds SeedSponsorsBootstrapper with demo
sponsor SVG assets and extends SeedCatalog with sponsor seed entries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Backend/Data/SeedDataBootstrapper.cs

@@ -22,6 +22,7 @@ public static partial class SeedDataBootstrapper
         if (seasons.TryGetValue(2026, out var currentSeason))
         {
             await EnsureCandidatesAsync(db, currentSeason, SeedCatalog.CurrentCandidateSeeds);
+            await EnsureSponsorsAsync(db, currentSeason);
             await EnsureSeedOperationalDataAsync(db, currentSeason);
         }