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 {

src/lib/types/app.ts

@@ -122,6 +122,12 @@ export interface SettingsState {
    * has effect when `nudgesEnabled` is true.
    */
   nudgesSpeakAloud: boolean;
+  /**
+   * Phase 8. Controls the 7-day momentum sparkline only. The "N today"
+   * badge on the Tasks header is always on regardless. Default true so
+   * existing users see the sparkline on upgrade.
+   */
+  showMomentumSparkline: boolean;
 }
 
 export interface Profile {
@@ -224,6 +230,15 @@ export interface TranscriptMetaPatch {
  */
 export type EnergyLevel = "high" | "medium" | "brain_dead";
 
+/**
+ * Phase 8: one bucket in the 7-day completion history fed to the
+ * momentum sparkline. `day` is "YYYY-MM-DD" local time.
+ */
+export interface DailyCompletionCount {
+  day: string;   // "YYYY-MM-DD" local
+  count: number;
+}
+
 export interface TaskDto {
   id: string;
   text: string;