feat(gamification): completionStats store

Owns the last-7-days completion series. Refreshes on task-completed /
step-completed / task-uncompleted / task-deleted window events and on
window focus (for day rollover). Derives todayCount from the newest
entry.

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

src/lib/stores/completionStats.svelte.ts

@@ -0,0 +1,56 @@
+// Phase 8 store for forgiving gamification. Owns the last-7-days series
+// of completion counts rendered by the Tasks-page badge + sparkline.
+//
+// Refresh triggers:
+//  - module load (first time)
+//  - kon:task-completed  (from page.svelte.ts completeTask)
+//  - kon:step-completed  (from MicroSteps.svelte)
+//  - kon:task-uncompleted (new event, emitted by uncompleteTask in Task 10)
+//  - kon:task-deleted    (new event, emitted by deleteTask in Task 10)
+//  - window focus        (for day rollover while the app stayed open
+//                         past midnight)
+//
+// We deliberately avoid polling. Every path that changes a row's
+// done state or deletes a row emits one of the events above.
+
+import { invoke } from "@tauri-apps/api/core";
+import { hasTauriRuntime } from "$lib/utils/runtime.js";
+import type { DailyCompletionCount } from "$lib/types/app";
+
+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);
+
+export async function refresh(): Promise<void> {
+  if (!hasTauriRuntime()) return;
+  try {
+    const series = await invoke<DailyCompletionCount[]>(
+      "list_recent_completions_cmd",
+      { days: DAYS },
+    );
+    recentCompletions.splice(0, recentCompletions.length, ...series);
+  } catch (err) {
+    // Do not toast. This is a background read. Log only.
+    console.error("completionStats.refresh failed", err);
+  }
+}
+
+if (typeof window !== "undefined") {
+  const handler = () => {
+    refresh().catch(() => {});
+  };
+  window.addEventListener("kon:task-completed", handler);
+  window.addEventListener("kon:step-completed", handler);
+  window.addEventListener("kon:task-uncompleted", handler);
+  window.addEventListener("kon:task-deleted", handler);
+  window.addEventListener("focus", handler);
+
+  if (hasTauriRuntime()) {
+    refresh().catch(() => {});
+  }
+}