feat: add opt-out toggle for prestige bot announcements

Adds enablePrestigeAnnouncements to ProfileSettings (defaults to true).
The prestige route now checks this setting before posting the Discord
webhook, and the edit profile modal exposes a toggle in the Sounds &
Notifications section so players can opt out.

Closes #169

packages/types/src/interfaces/profileSettings.ts

@@ -48,11 +48,17 @@ interface ProfileSettings {
    * Whether browser system notifications are enabled.
    */
   enableNotifications: boolean;
+
+  /**
+   * Whether prestige milestones are announced in the Discord server.
+   */
+  enablePrestigeAnnouncements: boolean;
 }
 
 // eslint-disable-next-line @typescript-eslint/naming-convention -- intentional constant name
 const DEFAULT_PROFILE_SETTINGS: ProfileSettings = {
   enableNotifications:              false,
+  enablePrestigeAnnouncements:      true,
   enableSounds:                     false,
   numberFormat:                     "suffix",
   showAchievementsUnlocked:         true,