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-tauri/src/settings.rs

@@ -205,6 +205,9 @@ pub struct AppSettings {
     /// User has confirmed the failover toggle first-run notice
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub failover_confirmed: Option<bool>,
+    /// User has confirmed the first-run welcome notice
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub first_run_notice_confirmed: Option<bool>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub language: Option<String>,
 
@@ -297,6 +300,7 @@ impl Default for AppSettings {
             stream_check_confirmed: None,
             enable_failover_toggle: false,
             failover_confirmed: None,
+            first_run_notice_confirmed: None,
             language: None,
             visible_apps: None,
             claude_config_dir: None,