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>

backend/src/app/seeds/loader.py

@@ -270,6 +270,7 @@ async def upsert_bosses(
                 pokemon_id=pokemon_id,
                 level=bp["level"],
                 order=bp["order"],
+                condition_label=bp.get("condition_label"),
             ))
 
         count += 1