implement sticker browser, pack management ui, wire up messages view controller

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

StickerCloner/StickerCloner MessagesExtension/StickerBrowserViewController.swift

@@ -0,0 +1,21 @@
+import Messages
+import UIKit
+
+final class StickerBrowserViewController: MSStickerBrowserViewController {
+	private var stickers: [MSSticker] = []
+
+	func reloadStickers() {
+		stickers = StickerStore.shared.loadStickers()
+		stickerBrowserView.reloadData()
+	}
+
+	// MARK: - MSStickerBrowserViewDataSource
+
+	override func numberOfStickers(in stickerBrowserView: MSStickerBrowserView) -> Int {
+		stickers.count
+	}
+
+	override func stickerBrowserView(_ stickerBrowserView: MSStickerBrowserView, stickerAt index: Int) -> MSSticker {
+		stickers[index]
+	}
+}