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/services/mod.rs

@@ -15,7 +15,8 @@ pub use mcp::McpService;
 pub use prompt::PromptService;
 pub use provider::{ProviderService, ProviderSortUpdate};
 pub use proxy::ProxyService;
-pub use skill::{Skill, SkillRepo, SkillService};
+#[allow(unused_imports)]
+pub use skill::{DiscoverableSkill, Skill, SkillRepo, SkillService};
 pub use speedtest::{EndpointLatency, SpeedtestService};
 #[allow(unused_imports)]
 pub use usage_stats::{