Add conditional boss battle teams (variant teams by condition)

Wire up the existing condition_label column on boss_pokemon to support
variant teams throughout the UI. Boss battles can now have multiple team
configurations based on conditions (e.g., starter choice in Gen 1).

- Add condition_label to BossPokemonInput schema (frontend + backend bulk import)
- Rewrite BossTeamEditor with variant tabs (Default + named conditions)
- Add variant pill selector to BossDefeatModal team preview
- Add BossTeamPreview component to RunEncounters boss cards
- Fix MissingGreenlet error in set_boss_team via session.expunge_all()
- Fix PokemonSelector state bleed between tabs via composite React key
- Add Alembic migration for condition_label column

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

frontend/src/types/admin.ts

@@ -176,4 +176,5 @@ export interface BossPokemonInput {
   pokemonId: number
   level: number
   order: number
+  conditionLabel?: string | null
 }

frontend/src/types/game.ts

@@ -137,6 +137,7 @@ export interface BossPokemon {
   pokemonId: number
   level: number
   order: number
+  conditionLabel: string | null
   pokemon: Pokemon
 }