refactor(backend): replace unsafe unwrap calls with proper error handling

- Add to_json_string helper for safe JSON serialization
- Add lock_conn macro for safe Mutex locking
- Replace 41 unwrap() calls with proper error handling:
  - database.rs: JSON serialization and Mutex operations (31 fixes)
  - lib.rs: macOS NSWindow and tray icon handling (3 fixes)
  - services/provider.rs: Claude model normalization (1 fix)
  - services/prompt.rs: timestamp generation (3 fixes)
  - services/skill.rs: directory name extraction (2 fixes)
  - mcp.rs: HashMap initialization and type conversions (5 fixes)
  - app_config.rs: timestamp fallback (1 fix)

This improves application stability and prevents potential panics.

src-tauri/src/services/skill.rs

@@ -231,7 +231,12 @@ impl SkillService {
             // 解析技能元数据
             match self.parse_skill_metadata(&skill_md) {
                 Ok(meta) => {
-                    let directory = path.file_name().unwrap().to_string_lossy().to_string();
+                    // 安全地获取目录名
+                    let Some(dir_name) = path.file_name() else {
+                        log::warn!("Failed to get directory name from path: {path:?}");
+                        continue;
+                    };
+                    let directory = dir_name.to_string_lossy().to_string();
 
                     // 构建 README URL（考虑 skillsPath）
                     let readme_path = if let Some(ref skills_path) = repo.skills_path {
@@ -305,7 +310,12 @@ impl SkillService {
                 continue;
             }
 
-            let directory = path.file_name().unwrap().to_string_lossy().to_string();
+            // 安全地获取目录名
+            let Some(dir_name) = path.file_name() else {
+                log::warn!("Failed to get directory name from path: {path:?}");
+                continue;
+            };
+            let directory = dir_name.to_string_lossy().to_string();
 
             // 更新已安装状态
             let mut found = false;