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/export.py

@@ -154,6 +154,7 @@ async def export_game_bosses(
                         "pokemon_name": bp.pokemon.name,
                         "level": bp.level,
                         "order": bp.order,
+                        **({"condition_label": bp.condition_label} if bp.condition_label else {}),
                     }
                     for bp in sorted(b.pokemon, key=lambda p: p.order)
                 ],