feat(gamification): today count + sparkline on Tasks header

Badge renders when today's count > 0. Sparkline renders when the
setting is enabled and any of the last 7 days has a completion.
Wrapped in a narrow aria-live region so increments announce without
re-reading the rest of the header.

Fix: converted todayCount from $derived module export to a getter
function (Svelte 5 derived_invalid_export constraint).

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

src/lib/pages/TasksPage.svelte

@@ -8,8 +8,10 @@
     taskLists, addTaskList, renameTaskList, deleteTaskList,
     settings, saveSettings,
   } from "$lib/stores/page.svelte.js";
+  import { recentCompletions, todayCount } from "$lib/stores/completionStats.svelte";
   import WipTaskList from '$lib/components/WipTaskList.svelte';
   import EmptyState from '$lib/components/EmptyState.svelte';
+  import CompletionSparkline from "$lib/components/CompletionSparkline.svelte";
   import EnergyChip from '$lib/components/EnergyChip.svelte';
   import { SquareCheck, Search, ExternalLink, ChevronLeft, ArrowUpDown, Plus, X, ChevronRight, Zap } from 'lucide-svelte';
   import Card from "$lib/components/Card.svelte";
@@ -284,7 +286,31 @@
   <!-- Header -->
   <div class="flex items-center px-7 pt-6 pb-2">
     <div>
-      <h2 class="text-xl font-display text-text italic">Tasks</h2>
+      <div class="flex items-baseline gap-3">
+        <h2 class="text-xl font-display text-text italic">Tasks</h2>
+
+        <!-- Phase 8 badge + sparkline. aria-live scoped to just this
+             wrapper so screen readers announce completions without
+             re-reading the rest of the header. -->
+        <div
+          class="flex items-center gap-2"
+          aria-live="polite"
+        >
+          {#if todayCount() > 0}
+            <span
+              class="text-[11px] text-text-tertiary"
+              aria-label={`${todayCount()} ${todayCount() === 1 ? "task" : "tasks"} completed today`}
+            >
+              {todayCount()} today
+            </span>
+          {/if}
+
+          {#if settings.showMomentumSparkline}
+            <CompletionSparkline data={recentCompletions} />
+          {/if}
+        </div>
+      </div>
+
       <p class="text-[11px] text-text-tertiary mt-1">Add tasks manually. Automatic extraction from your transcripts is coming.</p>
     </div>
     <div class="flex-1"></div>

src/lib/stores/completionStats.svelte.ts

@@ -21,10 +21,13 @@ const DAYS = 7;
 
 export const recentCompletions = $state<DailyCompletionCount[]>([]);
 
-// Reactive derived value. Svelte 5 template consumers read it as a
-// plain property (no parens) and it recomputes when recentCompletions
-// changes.
-export const todayCount = $derived(recentCompletions.at(-1)?.count ?? 0);
+// Reactive getter. Svelte 5 does not allow exporting $derived at module
+// level ("derived_invalid_export"), so we expose a function instead.
+// Template consumers call todayCount() in reactive contexts and it
+// recomputes when recentCompletions changes.
+export function todayCount(): number {
+  return recentCompletions.at(-1)?.count ?? 0;
+}
 
 export async function refresh(): Promise<void> {
   if (!hasTauriRuntime()) return;