팬덤 탈퇴 · 성운 창립자 권한 승계 · 대표 팬덤 자동 지정

app/lib/features/planet/planet_data.dart

@@ -329,6 +329,9 @@ abstract interface class PlanetRepository {
   /// 이 행성/성운에 가입 — `fn-fandom-join`. 이미 멤버면 서버가 아무것도 하지 않는다.
   Future<void> join();
 
+  /// 탈퇴 — `fn-fandom-leave`. 대표 팬덤 자동 지정(C-101) · 성운 창립자 승계(C-100)는 서버가 한다.
+  Future<void> leave();
+
   /// 명세 — *최초 5개, 스크롤 시 5개씩 호출*.
   Future<List<LoungePost>> morePosts({required int offset, int limit = 5});
 
@@ -391,6 +394,9 @@ class SamplePlanetRepository implements PlanetRepository {
   @override
   Future<void> join() async {}
 
+  @override
+  Future<void> leave() async {}
+
   @override
   Future<PlanetHistoryPage> history({String? cursor, bool personal = false}) async => const PlanetHistoryPage(items: []);
   @override

app/lib/features/planet/planet_detail_screen.dart

@@ -6,6 +6,7 @@ import '../../core/data/backend.dart' show BackendException;
 import '../../core/data/planet_visual_state.dart';
 
 import '../../core/theme/tokens.dart';
+import '../../core/ui/app_dialog.dart';
 import '../../core/ui/app_icons.dart';
 import '../eclipse/eclipse_data.dart';
 import '../eclipse/widgets/eclipse_banner.dart';
@@ -74,6 +75,30 @@ class _PlanetDetailScreenState extends State<PlanetDetailScreen> {
     }
   }
 
+  /// 「가입중」 알약 → 탈퇴 시트 → 확인 창 — Figma `343:213428`(탈퇴하기 · 취소) ·
+  /// `343:213582`(행성 탈퇴를 할까요?). 대표 팬덤 자동 지정 · 성운 창립자 승계는 서버 몫이다(C-100 · C-101).
+  void _leave() => showActionSheet(context, [
+    ActionSheetItem(context.l10n.planetLeave, () async {
+      final ok = await showConfirmDialog(
+        context,
+        context.l10n.planetLeaveConfirmTitle,
+        body: context.l10n.planetLeaveConfirmBody,
+        confirmLabel: context.l10n.commonConfirm,
+      );
+      if (!ok) return;
+      try {
+        await widget.repository.leave();
+        if (mounted) {
+          setState(() { _detail = widget.repository.load(nebula: widget.nebula); });
+        }
+      } on BackendException catch (e) {
+        if (mounted) {
+          ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(e.message)));
+        }
+      }
+    }),
+  ]);
+
   @override
   Widget build(BuildContext context) => FutureBuilder<PlanetDetail>(
     future: _detail,
@@ -127,7 +152,7 @@ class _PlanetDetailScreenState extends State<PlanetDetailScreen> {
                       ownerName: d.ownerName,
                       ownerPhoto: d.ownerPhoto,
                       joined: d.joined,
-                      onJoin: d.joined ? null : _join,
+                      onJoin: d.joined ? _leave : _join,
                       onOpenHistory: () => Navigator.of(context).push(
                         MaterialPageRoute<void>(
                           builder: (_) => PlanetHistoryScreen(

app/lib/features/planet/planet_repository_firestore.dart

@@ -103,6 +103,9 @@ class FirestorePlanetRepository implements PlanetRepository {
   @override
   Future<void> join() => _backend.call('fnFandomJoin', {'fandomId': fandomId});
 
+  @override
+  Future<void> leave() => _backend.call('fnFandomLeave', {'fandomId': fandomId});
+
   @override
   Future<PlanetDetail> load({required bool nebula}) async {
     final db = _backend.db;

app/lib/features/planet/widgets/planet_detail_hero.dart

@@ -43,7 +43,7 @@ class PlanetDetailHero extends StatelessWidget {
   final bool joined;
   final VoidCallback? onHeroParticipate;
 
-  /// 「가입하기」 알약을 눌렀을 때. 이미 가입했으면 null 이고 알약은 상태만 보인다.
+  /// 알약을 눌렀을 때 — 「가입하기」면 가입, 「가입중」이면 탈퇴 시트. null 이면 상태만 보인다.
   final VoidCallback? onJoin;
 
   /// 성운 상세(`373:442708`)는 규모 칩과 FPI 줄이 없다.