feat(migration): enable silent JSON to SQLite migration with toast notification

- Remove environment variable feature gate (CC_SWITCH_ENABLE_JSON_DB_MIGRATION)
- Automatically migrate config.json to SQLite when db doesn't exist
- Archive migrated config.json as config.json.migrated for recovery
- Add migration success flag in init_status.rs (one-time consumption)
- Add get_migration_result command for frontend to query
- Show toast notification on successful migration in App.tsx

src-tauri/src/init_status.rs

@@ -25,6 +25,33 @@ pub fn get_init_error() -> Option<InitErrorPayload> {
     cell().read().ok()?.clone()
 }
 
+// ============================================================
+// 迁移结果状态
+// ============================================================
+
+static MIGRATION_SUCCESS: OnceLock<RwLock<bool>> = OnceLock::new();
+
+fn migration_cell() -> &'static RwLock<bool> {
+    MIGRATION_SUCCESS.get_or_init(|| RwLock::new(false))
+}
+
+pub fn set_migration_success() {
+    if let Ok(mut guard) = migration_cell().write() {
+        *guard = true;
+    }
+}
+
+/// 获取并消费迁移成功状态（只返回一次 true，之后返回 false）
+pub fn take_migration_success() -> bool {
+    if let Ok(mut guard) = migration_cell().write() {
+        let val = *guard;
+        *guard = false;
+        val
+    } else {
+        false
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;