fix(backup): restrict SQL import to CC Switch exported backups only

- Add validation to reject SQL files without CC Switch export header
- Remove redundant sanitize_import_sql (sqlite_* objects already excluded at export time)
- Fix backup filename collision by appending counter suffix
- Update i18n hints to clarify import restriction

src-tauri/tests/import_export_sync.rs

@@ -1003,3 +1003,76 @@ fn export_sql_returns_error_for_invalid_path() {
         other => panic!("expected IoContext or Io error, got {other:?}"),
     }
 }
+
+#[test]
+fn import_sql_rejects_non_cc_switch_backup() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+    let home = ensure_test_home();
+
+    let state = create_test_state().expect("create test state");
+
+    let import_path = home.join("not-cc-switch.sql");
+    fs::write(&import_path, "CREATE TABLE x (id INTEGER);").expect("write import sql");
+
+    let err = state
+        .db
+        .import_sql(&import_path)
+        .expect_err("non-cc-switch sql should be rejected");
+
+    match err {
+        AppError::Localized { key, .. } => {
+            assert_eq!(key, "backup.sql.invalid_format");
+        }
+        other => panic!("expected Localized error, got {other:?}"),
+    }
+}
+
+#[test]
+fn import_sql_accepts_cc_switch_exported_backup() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+    let home = ensure_test_home();
+
+    // Create a database with some data and export it.
+    let mut config = MultiAppConfig::default();
+    {
+        let manager = config
+            .get_manager_mut(&AppType::Claude)
+            .expect("claude manager");
+        manager.current = "test-provider".to_string();
+        manager.providers.insert(
+            "test-provider".to_string(),
+            Provider::with_id(
+                "test-provider".to_string(),
+                "Test Provider".to_string(),
+                json!({"env": {"ANTHROPIC_API_KEY": "test-key"}}),
+                None,
+            ),
+        );
+    }
+
+    let state = create_test_state_with_config(&config).expect("create test state");
+    let export_path = home.join("cc-switch-export.sql");
+    state
+        .db
+        .export_sql(&export_path)
+        .expect("export should succeed");
+
+    // Reset database, then import into a fresh one.
+    reset_test_fs();
+    let state = create_test_state().expect("create test state");
+    state
+        .db
+        .import_sql(&export_path)
+        .expect("import should succeed");
+
+    let providers = state
+        .db
+        .get_all_providers(AppType::Claude.as_str())
+        .expect("load providers");
+    assert!(
+        providers.contains_key("test-provider"),
+        "imported providers should contain test-provider"
+    );
+}