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-tauri/src/database/mod.rs

@@ -47,7 +47,7 @@ const DB_BACKUP_RETAIN: usize = 10;
 
 /// 当前 Schema 版本号
 /// 每次修改表结构时递增，并在 schema.rs 中添加相应的迁移逻辑
-pub(crate) const SCHEMA_VERSION: i32 = 2;
+pub(crate) const SCHEMA_VERSION: i32 = 3;
 
 /// 安全地序列化 JSON，避免 unwrap panic
 pub(crate) fn to_json_string<T: Serialize>(value: &T) -> Result<String, AppError> {