refactor(database): split monolithic database.rs into modular structure

Split the 1788-line database.rs into a well-organized module directory:

- mod.rs (142 lines): Database struct and initialization
- schema.rs (341 lines): Table creation and schema migrations
- backup.rs (324 lines): SQL import/export and snapshot backup
- migration.rs (240 lines): JSON to SQLite data migration
- tests.rs (280 lines): Unit tests
- dao/providers.rs (254 lines): Provider CRUD operations
- dao/mcp.rs (97 lines): MCP server CRUD operations
- dao/prompts.rs (88 lines): Prompt CRUD operations
- dao/skills.rs (124 lines): Skills CRUD operations
- dao/settings.rs (65 lines): Settings key-value storage

Benefits:
- Max file size reduced from 1788 to 341 lines
- Clear separation of concerns (schema, backup, migration, DAOs)
- Easier to navigate and maintain
- All 107 tests passing with zero API changes

src-tauri/src/database/dao/skills.rs

@@ -0,0 +1,124 @@
+//! Skills 数据访问对象
+//!
+//! 提供 Skills 和 Skill Repos 的 CRUD 操作。
+
+use crate::database::{lock_conn, Database};
+use crate::error::AppError;
+use crate::services::skill::{SkillRepo, SkillState};
+use indexmap::IndexMap;
+use rusqlite::params;
+
+impl Database {
+    /// 获取所有 Skills 状态
+    pub fn get_skills(&self) -> Result<IndexMap<String, SkillState>, AppError> {
+        let conn = lock_conn!(self.conn);
+        let mut stmt = conn
+            .prepare("SELECT key, installed, installed_at FROM skills ORDER BY key ASC")
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let skill_iter = stmt
+            .query_map([], |row| {
+                let key: String = row.get(0)?;
+                let installed: bool = row.get(1)?;
+                let installed_at_ts: i64 = row.get(2)?;
+
+                let installed_at =
+                    chrono::DateTime::from_timestamp(installed_at_ts, 0).unwrap_or_default();
+
+                Ok((
+                    key,
+                    SkillState {
+                        installed,
+                        installed_at,
+                    },
+                ))
+            })
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let mut skills = IndexMap::new();
+        for skill_res in skill_iter {
+            let (key, skill) = skill_res.map_err(|e| AppError::Database(e.to_string()))?;
+            skills.insert(key, skill);
+        }
+        Ok(skills)
+    }
+
+    /// 更新 Skill 状态
+    pub fn update_skill_state(&self, key: &str, state: &SkillState) -> Result<(), AppError> {
+        let conn = lock_conn!(self.conn);
+        conn.execute(
+            "INSERT OR REPLACE INTO skills (key, installed, installed_at) VALUES (?1, ?2, ?3)",
+            params![key, state.installed, state.installed_at.timestamp()],
+        )
+        .map_err(|e| AppError::Database(e.to_string()))?;
+        Ok(())
+    }
+
+    /// 获取所有 Skill 仓库
+    pub fn get_skill_repos(&self) -> Result<Vec<SkillRepo>, AppError> {
+        let conn = lock_conn!(self.conn);
+        let mut stmt = conn
+            .prepare("SELECT owner, name, branch, enabled, skills_path FROM skill_repos ORDER BY owner ASC, name ASC")
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let repo_iter = stmt
+            .query_map([], |row| {
+                Ok(SkillRepo {
+                    owner: row.get(0)?,
+                    name: row.get(1)?,
+                    branch: row.get(2)?,
+                    enabled: row.get(3)?,
+                    skills_path: row.get(4)?,
+                })
+            })
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let mut repos = Vec::new();
+        for repo_res in repo_iter {
+            repos.push(repo_res.map_err(|e| AppError::Database(e.to_string()))?);
+        }
+        Ok(repos)
+    }
+
+    /// 保存 Skill 仓库
+    pub fn save_skill_repo(&self, repo: &SkillRepo) -> Result<(), AppError> {
+        let conn = lock_conn!(self.conn);
+        conn.execute(
+            "INSERT OR REPLACE INTO skill_repos (owner, name, branch, enabled, skills_path) VALUES (?1, ?2, ?3, ?4, ?5)",
+            params![repo.owner, repo.name, repo.branch, repo.enabled, repo.skills_path],
+        ).map_err(|e| AppError::Database(e.to_string()))?;
+        Ok(())
+    }
+
+    /// 删除 Skill 仓库
+    pub fn delete_skill_repo(&self, owner: &str, name: &str) -> Result<(), AppError> {
+        let conn = lock_conn!(self.conn);
+        conn.execute(
+            "DELETE FROM skill_repos WHERE owner = ?1 AND name = ?2",
+            params![owner, name],
+        )
+        .map_err(|e| AppError::Database(e.to_string()))?;
+        Ok(())
+    }
+
+    /// 初始化默认的 Skill 仓库（首次启动时调用）
+    pub fn init_default_skill_repos(&self) -> Result<usize, AppError> {
+        // 检查是否已有仓库
+        let existing = self.get_skill_repos()?;
+        if !existing.is_empty() {
+            return Ok(0);
+        }
+
+        // 获取默认仓库列表
+        let default_store = crate::services::skill::SkillStore::default();
+        let mut count = 0;
+
+        for repo in &default_store.repos {
+            self.save_skill_repo(repo)?;
+            count += 1;
+        }
+
+        log::info!("初始化默认 Skill 仓库完成，共 {count} 个");
+        Ok(count)
+    }
+}