restructure as standalone iOS app + iMessage extension

convert container from Messages-only app to regular iOS app,
move shared code (models, api client, store) to Shared/ group,
add app group entitlements for cross-process data sharing,
rewrite StickerStore for shared UserDefaults + container,
create SwiftUI app entry point, simplify extension to read-only browser

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

StickerCloner/Shared/StickerStore.swift

@@ -0,0 +1,116 @@
+import Foundation
+
+final class StickerStore {
+	static let shared = StickerStore()
+
+	private let packsKey = "savedPacks"
+	private let suiteName = "group.de.felixfoertsch.StickerCloner"
+	private let fileManager = FileManager.default
+
+	private lazy var defaults: UserDefaults = {
+		guard let defaults = UserDefaults(suiteName: suiteName) else {
+			fatalError("App Group '\(suiteName)' is not configured")
+		}
+		return defaults
+	}()
+
+	private init() {
+		migrateIfNeeded()
+	}
+
+	// MARK: - Pack persistence
+
+	var savedPacks: [SavedPack] {
+		get {
+			guard let data = defaults.data(forKey: packsKey) else { return [] }
+			return (try? JSONDecoder().decode([SavedPack].self, from: data)) ?? []
+		}
+		set {
+			let data = try? JSONEncoder().encode(newValue)
+			defaults.set(data, forKey: packsKey)
+		}
+	}
+
+	// MARK: - File management
+
+	private var stickersDirectory: URL {
+		let container = fileManager.containerURL(forSecurityApplicationGroupIdentifier: suiteName)!
+		return container.appendingPathComponent("stickers", isDirectory: true)
+	}
+
+	private func packDirectory(name: String) -> URL {
+		stickersDirectory.appendingPathComponent(name, isDirectory: true)
+	}
+
+	// MARK: - Add / remove packs
+
+	func addPack(response: StickerSetResponse) async throws {
+		let pack = SavedPack(name: response.name, title: response.title, stickerCount: response.stickerCount)
+
+		let packDir = packDirectory(name: response.name)
+		try fileManager.createDirectory(at: packDir, withIntermediateDirectories: true)
+
+		for sticker in response.stickers {
+			let localURL = packDir.appendingPathComponent("\(sticker.id).png")
+			if fileManager.fileExists(atPath: localURL.path) { continue }
+			try await StickerClonerAPI.downloadSticker(from: sticker.pngUrl, to: localURL)
+		}
+
+		var packs = savedPacks
+		packs.removeAll { $0.name == pack.name }
+		packs.append(pack)
+		savedPacks = packs
+	}
+
+	func removePack(name: String) {
+		var packs = savedPacks
+		packs.removeAll { $0.name == name }
+		savedPacks = packs
+
+		let packDir = packDirectory(name: name)
+		try? fileManager.removeItem(at: packDir)
+	}
+
+	// MARK: - Load sticker file URLs
+
+	func stickerFileURLs() -> [URL] {
+		var urls: [URL] = []
+
+		for pack in savedPacks {
+			let packDir = packDirectory(name: pack.name)
+			guard let files = try? fileManager.contentsOfDirectory(at: packDir, includingPropertiesForKeys: nil) else { continue }
+
+			for file in files.sorted(by: { $0.lastPathComponent < $1.lastPathComponent }) {
+				guard file.pathExtension == "png" else { continue }
+				urls.append(file)
+			}
+		}
+
+		return urls
+	}
+
+	// MARK: - Migration from old sandbox
+
+	private func migrateIfNeeded() {
+		let migrated = defaults.bool(forKey: "didMigrateToAppGroup")
+		guard !migrated else { return }
+
+		let oldDefaults = UserDefaults.standard
+		if let data = oldDefaults.data(forKey: packsKey) {
+			defaults.set(data, forKey: packsKey)
+			oldDefaults.removeObject(forKey: packsKey)
+		}
+
+		let oldCaches = fileManager.urls(for: .cachesDirectory, in: .userDomainMask).first!
+		let oldStickersDir = oldCaches.appendingPathComponent("stickers", isDirectory: true)
+		if fileManager.fileExists(atPath: oldStickersDir.path) {
+			let newStickersDir = stickersDirectory
+			try? fileManager.createDirectory(at: newStickersDir.deletingLastPathComponent(), withIntermediateDirectories: true)
+			if !fileManager.fileExists(atPath: newStickersDir.path) {
+				try? fileManager.moveItem(at: oldStickersDir, to: newStickersDir)
+			}
+		}
+
+		defaults.set(true, forKey: "didMigrateToAppGroup")
+	}
+}