feat: Add community management features with QR code scanning

- Implement Community model for managing community data, including secret handling and PSK derivation.
- Create CommunityQrScannerScreen for scanning and joining communities via QR codes.
- Develop CommunityStore for persisting community data using SharedPreferences.
- Introduce QrCodeDisplay widget for displaying QR codes with customizable options.
- Add QrScannerWidget for reusable QR code scanning functionality with validation and controls.

lib/storage/community_store.dart

@@ -0,0 +1,117 @@
+import 'dart:convert';
+
+import '../models/community.dart';
+import 'prefs_manager.dart';
+
+/// Persists communities to local storage using SharedPreferences.
+///
+/// Communities are stored as a JSON array under a single key.
+/// Each community contains its secret K, so this data should
+/// be considered sensitive (though device encryption handles security).
+class CommunityStore {
+  static const String _communitiesKey = 'communities_v1';
+
+  /// Load all communities from storage
+  Future<List<Community>> loadCommunities() async {
+    final prefs = PrefsManager.instance;
+    final jsonString = prefs.getString(_communitiesKey);
+    if (jsonString == null || jsonString.isEmpty) {
+      return [];
+    }
+
+    try {
+      final jsonList = jsonDecode(jsonString) as List<dynamic>;
+      return jsonList
+          .map((json) => Community.fromJson(json as Map<String, dynamic>))
+          .toList();
+    } catch (e) {
+      // If JSON is corrupted, return empty list
+      return [];
+    }
+  }
+
+  /// Save all communities to storage
+  Future<void> saveCommunities(List<Community> communities) async {
+    final prefs = PrefsManager.instance;
+    final jsonList = communities.map((c) => c.toJson()).toList();
+    await prefs.setString(_communitiesKey, jsonEncode(jsonList));
+  }
+
+  /// Add a new community
+  Future<void> addCommunity(Community community) async {
+    final communities = await loadCommunities();
+    
+    // Check if community with same ID already exists
+    final existingIndex = communities.indexWhere((c) => c.id == community.id);
+    if (existingIndex >= 0) {
+      // Replace existing
+      communities[existingIndex] = community;
+    } else {
+      communities.add(community);
+    }
+    
+    await saveCommunities(communities);
+  }
+
+  /// Update an existing community
+  Future<void> updateCommunity(Community community) async {
+    final communities = await loadCommunities();
+    final index = communities.indexWhere((c) => c.id == community.id);
+    if (index >= 0) {
+      communities[index] = community;
+      await saveCommunities(communities);
+    }
+  }
+
+  /// Remove a community by ID
+  Future<void> removeCommunity(String communityId) async {
+    final communities = await loadCommunities();
+    communities.removeWhere((c) => c.id == communityId);
+    await saveCommunities(communities);
+  }
+
+  /// Get a community by ID
+  Future<Community?> getCommunity(String communityId) async {
+    final communities = await loadCommunities();
+    try {
+      return communities.firstWhere((c) => c.id == communityId);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  /// Check if a community with the same secret already exists
+  /// (to prevent duplicate imports from QR scanning)
+  Future<Community?> findByCommunityId(String cid) async {
+    final communities = await loadCommunities();
+    try {
+      return communities.firstWhere((c) => c.communityId == cid);
+    } catch (_) {
+      return null;
+    }
+  }
+
+  /// Add a hashtag channel to a community
+  Future<void> addHashtagChannel(
+    String communityId,
+    String hashtag,
+  ) async {
+    final community = await getCommunity(communityId);
+    if (community != null) {
+      final updated = community.addHashtagChannel(hashtag);
+      await updateCommunity(updated);
+    }
+  }
+
+  /// Remove a hashtag channel from a community
+  Future<void> removeHashtagChannel(
+    String communityId,
+    String hashtag,
+  ) async {
+    final community = await getCommunity(communityId);
+    if (community != null) {
+      final updated = community.removeHashtagChannel(hashtag);
+      await updateCommunity(updated);
+    }
+  }
+}