feat(skills): unified management architecture with SSOT and React Query

- Introduce SSOT (Single Source of Truth) at ~/.cc-switch/skills/
- Add three-app toggle support (Claude/Codex/Gemini) for each skill
- Refactor frontend to use TanStack Query hooks instead of manual state
- Add UnifiedSkillsPanel for managing installed skills with app toggles
- Add useSkills.ts with declarative data fetching hooks
- Extend skills.ts API with unified install/uninstall/toggle methods
- Support importing unmanaged skills from app directories
- Add v2→v3 database migration for new skills table structure

src/i18n/locales/en.json

@@ -870,7 +870,25 @@
       "installed": "Installed",
       "uninstalled": "Not installed"
     },
-    "noResults": "No matching skills found"
+    "noResults": "No matching skills found",
+    "noInstalled": "No skills installed",
+    "noInstalledDescription": "Discover and install skills from repositories, or import existing skills",
+    "discover": "Discover Skills",
+    "import": "Import Existing",
+    "importDescription": "Select skills to import into CC Switch unified management",
+    "importSuccess": "Successfully imported {{count}} skills",
+    "importSelected": "Import Selected ({{count}})",
+    "noUnmanagedFound": "No skills to import found. All skills are already managed by CC Switch.",
+    "foundIn": "Found in",
+    "local": "Local",
+    "uninstallConfirm": "Are you sure you want to uninstall \"{{name}}\"? This will remove the skill from all apps.",
+    "uninstallInMainPanel": "Please uninstall skills from the main panel",
+    "notFound": "Skill not found",
+    "apps": {
+      "claude": "Claude",
+      "codex": "Codex",
+      "gemini": "Gemini"
+    }
   },
   "deeplink": {
     "confirmImport": "Confirm Import Provider",
@@ -958,7 +976,10 @@
     "clickToSelect": "Click to select icon"
   },
   "migration": {
-    "success": "Configuration migrated successfully"
+    "success": "Configuration migrated successfully",
+    "skillsSuccess": "Automatically imported {{count}} skill(s) into unified management",
+    "skillsFailed": "Failed to auto import skills",
+    "skillsFailedDescription": "Open the Skills page and click \"Import Existing\" to import manually (or restart and try again)."
   },
   "agents": {
     "title": "Agents"

src/i18n/locales/ja.json

@@ -870,7 +870,25 @@
       "installed": "インストール済み",
       "uninstalled": "未インストール"
     },
-    "noResults": "一致するスキルが見つかりませんでした"
+    "noResults": "一致するスキルが見つかりませんでした",
+    "noInstalled": "インストールされたスキルがありません",
+    "noInstalledDescription": "リポジトリからスキルを発見してインストールするか、既存のスキルをインポートしてください",
+    "discover": "スキルを発見",
+    "import": "既存をインポート",
+    "importDescription": "CC Switch 統合管理にインポートするスキルを選択してください",
+    "importSuccess": "{{count}} 件のスキルをインポートしました",
+    "importSelected": "選択をインポート ({{count}})",
+    "noUnmanagedFound": "インポートするスキルが見つかりませんでした。すべてのスキルは CC Switch で管理されています。",
+    "foundIn": "発見場所",
+    "local": "ローカル",
+    "uninstallConfirm": "「{{name}}」をアンインストールしますか？すべてのアプリからこのスキルが削除されます。",
+    "uninstallInMainPanel": "メインパネルからスキルをアンインストールしてください",
+    "notFound": "スキルが見つかりません",
+    "apps": {
+      "claude": "Claude",
+      "codex": "Codex",
+      "gemini": "Gemini"
+    }
   },
   "deeplink": {
     "confirmImport": "プロバイダーのインポートを確認",
@@ -958,7 +976,10 @@
     "clickToSelect": "クリックでアイコンを選択"
   },
   "migration": {
-    "success": "設定の移行が完了しました"
+    "success": "設定の移行が完了しました",
+    "skillsSuccess": "スキルを {{count}} 件、自動的に統合管理へインポートしました",
+    "skillsFailed": "スキルの自動インポートに失敗しました",
+    "skillsFailedDescription": "Skills 画面で「既存をインポート」をクリックして手動でインポートしてください（または再起動して再試行）。"
   },
   "agents": {
     "title": "エージェント"

src/i18n/locales/zh.json

@@ -870,7 +870,25 @@
       "installed": "已安装",
       "uninstalled": "未安装"
     },
-    "noResults": "未找到匹配的技能"
+    "noResults": "未找到匹配的技能",
+    "noInstalled": "暂无已安装的技能",
+    "noInstalledDescription": "从仓库发现并安装技能，或导入已有的技能",
+    "discover": "发现技能",
+    "import": "导入已有",
+    "importDescription": "选择要导入到 CC Switch 统一管理的技能",
+    "importSuccess": "成功导入 {{count}} 个技能",
+    "importSelected": "导入已选 ({{count}})",
+    "noUnmanagedFound": "未发现需要导入的技能。所有技能已在 CC Switch 统一管理中。",
+    "foundIn": "发现于",
+    "local": "本地",
+    "uninstallConfirm": "确定要卸载技能 \"{{name}}\" 吗？这将从所有应用中移除该技能。",
+    "uninstallInMainPanel": "请在主面板中卸载技能",
+    "notFound": "未找到技能",
+    "apps": {
+      "claude": "Claude",
+      "codex": "Codex",
+      "gemini": "Gemini"
+    }
   },
   "deeplink": {
     "confirmImport": "确认导入供应商配置",
@@ -958,7 +976,10 @@
     "clickToSelect": "点击选择图标"
   },
   "migration": {
-    "success": "配置迁移成功"
+    "success": "配置迁移成功",
+    "skillsSuccess": "已自动导入 {{count}} 个技能到统一管理",
+    "skillsFailed": "自动导入技能失败",
+    "skillsFailedDescription": "请打开 Skills 页面点击“导入已有”手动导入（或重启后再试）。"
   },
   "agents": {
     "title": "智能体"