feat: add first-run confirmation dialog for stream check

Show an informational dialog when users first click the health check
button, explaining its limitations (OAuth providers, relay services,
Bedrock). The dialog persists the confirmation in settings so it only
appears once per device.

src-tauri/src/settings.rs

@@ -196,6 +196,9 @@ pub struct AppSettings {
     /// User has confirmed the usage query first-run notice
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub usage_confirmed: Option<bool>,
+    /// User has confirmed the stream check first-run notice
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub stream_check_confirmed: Option<bool>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub language: Option<String>,
 
@@ -282,6 +285,7 @@ impl Default for AppSettings {
             enable_local_proxy: false,
             proxy_confirmed: None,
             usage_confirmed: None,
+            stream_check_confirmed: None,
             language: None,
             visible_apps: None,
             claude_config_dir: None,