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/provider.rs

@@ -424,9 +424,9 @@ impl ProviderService {
     /// 归一化 Claude 模型键：读旧键(ANTHROPIC_SMALL_FAST_MODEL)，写新键(DEFAULT_*), 并删除旧键
     fn normalize_claude_models_in_value(settings: &mut Value) -> bool {
         let mut changed = false;
-        let env = match settings.get_mut("env") {
-            Some(v) if v.is_object() => v.as_object_mut().unwrap(),
-            _ => return changed,
+        let env = match settings.get_mut("env").and_then(|v| v.as_object_mut()) {
+            Some(obj) => obj,
+            None => return changed,
         };
 
         let model = env