feat(gamification): DailyCompletionCount type + showMomentumSparkline setting

Adds the Phase 8 frontend types. New setting defaults to true (sparkline
on for everyone on upgrade) and persists via the existing settings
envelope; no migration needed because missing keys spread over the
defaults object on load.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/stores/page.svelte.ts

@@ -82,6 +82,7 @@ const defaults: SettingsState = {
   nudgesEnabled: false,
   nudgesMuted: false,
   nudgesSpeakAloud: false,
+  showMomentumSparkline: true,
 };
 
 function canUseStorage(): boolean {