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/api/bosses.py

@@ -228,11 +228,17 @@ async def set_boss_team(
             pokemon_id=item.pokemon_id,
             level=item.level,
             order=item.order,
+            condition_label=item.condition_label,
         )
         session.add(bp)
 
     await session.commit()
 
+    # Clear identity map so selectinload fetches everything fresh
+    # (expired Pokemon from deleted BossPokemon would otherwise cause
+    # MissingGreenlet errors during response serialization)
+    session.expunge_all()
+
     # Re-fetch with eager loading
     result = await session.execute(
         select(BossBattle)