feat: 添加 Anki APKG 导入/导出功能

- 添加 APKG 解析器 (src/lib/anki/apkg-parser.ts)
- 添加 APKG 导出器 (src/lib/anki/apkg-exporter.ts)
- 添加导入/导出 Server Actions
- 添加导入/导出 UI 组件
- 集成到牌组页面
- 添加 i18n 翻译

同时修复断链:
- /folders → /decks (Navbar, signup, profile)

messages/en-US.json

@@ -456,6 +456,60 @@
       "view": "View"
     }
   },
+  "decks": {
+    "title": "Decks",
+    "subtitle": "Manage your flashcard decks",
+    "newDeck": "New Deck",
+    "enterDeckName": "Enter deck name:",
+    "confirmDelete": "Type \"{name}\" to delete:",
+    "noDecksYet": "No decks yet",
+    "deckInfo": "ID: {id} • {totalCards} cards",
+    "loading": "Loading...",
+    "public": "Public",
+    "private": "Private",
+    "setPublic": "Set Public",
+    "setPrivate": "Set Private",
+    "enterNewName": "Enter new name:",
+    "importApkg": "Import APKG",
+    "exportApkg": "Export APKG",
+    "clickToUpload": "Click to upload .apkg file",
+    "apkgFilesOnly": "APKG files only",
+    "parsing": "Parsing file...",
+    "foundDecks": "Found {count} deck(s)",
+    "deckName": "Deck Name",
+    "back": "Back",
+    "import": "Import",
+    "importing": "Importing...",
+    "exportSuccess": "Deck exported successfully",
+    "goToDecks": "Go to Decks"
+  },
+  "decks": {
+    "title": "Decks",
+    "subtitle": "Manage your flashcard decks",
+    "newDeck": "New Deck",
+    "noDecksYet": "No decks yet",
+    "loading": "Loading...",
+    "deckInfo": "ID: {id} • {totalCards} cards",
+    "enterDeckName": "Enter deck name:",
+    "enterNewName": "Enter new name:",
+    "confirmDelete": "Type \"{name}\" to delete:",
+    "public": "Public",
+    "private": "Private",
+    "setPublic": "Set Public",
+    "setPrivate": "Set Private",
+    "importApkg": "Import APKG",
+    "exportApkg": "Export APKG",
+    "clickToUpload": "Click to upload an APKG file",
+    "apkgFilesOnly": "Only .apkg files are supported",
+    "parsing": "Parsing...",
+    "foundDecks": "Found {count} deck(s)",
+    "deckName": "Deck Name",
+    "back": "Back",
+    "import": "Import",
+    "importing": "Importing...",
+    "exportSuccess": "Deck exported successfully",
+    "goToDecks": "Go to Decks"
+  },
   "follow": {
     "follow": "Follow",
     "following": "Following",