feat(welcome): show first-run welcome dialog on fresh install

Introduce a one-time welcome dialog that explains CC Switch's workflow
to new users: how their existing config is preserved as a "default"
provider and how the bundled "Official" preset enables one-click revert.
Upgrade users are excluded by checking is_providers_empty() at startup
and never see the dialog.

Persistence follows the existing *_confirmed convention in AppSettings
(proxy/usage/stream_check/failover), stored in settings.json. The field
is only written when the user explicitly clicks the confirm button,
keeping its semantics strictly about user acknowledgement.

Also adds two reusable DAO helpers:
- Database::is_providers_empty for fresh-install detection, using
  EXISTS(SELECT 1) for a short-circuit query.
- Database::get_bool_flag accepting "true" | "1", with
  init_default_official_providers migrated to use it.

Dialog copy in zh/en/ja uses conditional phrasing so it stays
accurate whether or not existing live config was found.

src/types.ts

@@ -264,6 +264,8 @@ export interface Settings {
   enableFailoverToggle?: boolean;
   // User has confirmed the failover toggle first-run notice
   failoverConfirmed?: boolean;
+  // User has confirmed the first-run welcome notice
+  firstRunNoticeConfirmed?: boolean;
   // User has confirmed the auto-sync traffic warning
   autoSyncConfirmed?: boolean;
   // 首选语言（可选，默认中文）