Add killerName to Mission model and enhance overlay components

- Introduced killerName field in the Mission model within the Prisma schema and created a corresponding migration.
- Updated mission management logic to assign a killerName during mission creation.
- Enhanced overlay components to display killer information, including avatar and name, in both expanded and minimized panels.
- Added new CSS styles for killer display in the overlay.
- Included new avatar images for various killers in the overlay assets.

apps/api/prisma/migrations/20260511000000_add_mission_killer_name/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "missions" ADD COLUMN "killer_name" TEXT;

apps/api/prisma/schema.prisma

@@ -39,6 +39,7 @@ model Mission {
   difficulty              Int                  @db.SmallInt
   durationMinutes         Int                  @default(20) @map("duration_minutes") @db.SmallInt
   status                  String               @default("active")
+  killerName              String?              @map("killer_name")
   encounterLibraryVersion String               @map("encounter_library_version")
   startedAt               DateTime             @default(now()) @map("started_at")
   endedAt                 DateTime?            @map("ended_at")