feat(gamification): CompletionSparkline component

Tiny inline SVG. Seven bars, zero-days render as 1 px baseline stubs.
fill=currentColor so the parent's text colour (tertiary ink on the
Tasks header) drives it. Self-hides if all 7 days are zero.

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

src/lib/components/CompletionSparkline.svelte

@@ -0,0 +1,59 @@
+<script lang="ts">
+  // Phase 8. Tiny inline SVG sparkline. 7 bars, oldest to newest.
+  // Zero-days render as a 1 px baseline stub (never a gap, never zero
+  // height). The component self-hides when all bars are zero. Caller
+  // still guards on the user's settings toggle.
+  import type { DailyCompletionCount } from "$lib/types/app";
+
+  interface Props {
+    data: DailyCompletionCount[];
+    /** Total width in px. Default 80. */
+    width?: number;
+    /** Total height in px. Default 16. */
+    height?: number;
+  }
+
+  let { data, width = 80, height = 16 }: Props = $props();
+
+  const BAR_GAP = 2;
+
+  let maxCount = $derived(Math.max(1, ...data.map((d) => d.count)));
+  let barWidth = $derived(
+    (width - BAR_GAP * Math.max(0, data.length - 1)) / Math.max(1, data.length),
+  );
+
+  let ariaLabel = $derived.by(() => {
+    if (data.length === 0) return "";
+    const nums = data.map((d) => d.count).join(", ");
+    return `Tasks completed over the last ${data.length} days: ${nums}`;
+  });
+
+  let hasAnyCompletion = $derived(data.some((d) => d.count > 0));
+</script>
+
+{#if hasAnyCompletion}
+  <svg
+    {width}
+    {height}
+    viewBox={`0 0 ${width} ${height}`}
+    role="img"
+    aria-label={ariaLabel}
+    class="text-text-tertiary"
+  >
+    {#each data as d, i}
+      {@const x = i * (barWidth + BAR_GAP)}
+      {@const proportion = d.count / maxCount}
+      {@const barHeight = Math.max(1, Math.round(proportion * height))}
+      {@const y = height - barHeight}
+      <rect
+        {x}
+        {y}
+        width={barWidth}
+        height={barHeight}
+        fill="currentColor"
+        opacity={d.count === 0 ? 0.35 : 0.85}
+        rx="1"
+      />
+    {/each}
+  </svg>
+{/if}