feat(gamification): settings toggle for momentum sparkline

Default on. Controls only the sparkline; the "N today" badge is
unconditional. Copy kept in the zero-loss register: "Never counts
against you." Co-located with the Rituals section for Phase 8.
Can move to a dedicated section in Phase 9 polish.

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

src/lib/pages/SettingsPage.svelte

@@ -1529,6 +1529,14 @@
                 </div>
               </div>
             </div>
+
+            <!-- Phase 8: forgiving gamification. Toggle controls the sparkline only.
+                 The "N today" badge on the Tasks header is always on. -->
+            <Toggle
+              bind:checked={settings.showMomentumSparkline}
+              label="Show momentum sparkline"
+              description="A tiny chart of the last 7 days' completion counts, shown on the Tasks header. Never counts against you."
+            />
           </div>
         {/if}
       </div>