activity-stats: shared StatRow + canonical metrics across surfaces

Implements the activity-stats change (specs/activity-stats):

- gpx: `movingTime(tracks)` — moving seconds from trackpoint timestamps,
  excluding stationary spans + long gaps; null when no timestamps.
- stats.ts: pure formatters (distance/elevation/duration/speed/pace),
  sport-aware `deriveRate` (pace for foot sports, speed otherwise, speed
  default), and `activityStatItems` encoding the canonical order
  (distance · time · [moving] · pace/speed · ascent · descent; compact subset).
- StatRow: one shared presentational component (size sm/lg), adopted on the
  activity detail (full set; loader derives moving time), route detail
  (distance/ascent/descent), feed card + profile list (compact).
- i18n: journal.stats.* in en + de.

Tests: movingTime (stationary/gap exclusion, moving ≤ elapsed), deriveRate,
formatters, activityStatItems ordering + compact, StatRow render (jsdom).
typecheck + lint + unit (gpx 63, journal 311) green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/journal/app/components/StatRow.test.tsx

@@ -0,0 +1,29 @@
+// @vitest-environment jsdom
+import { describe, it, expect, afterEach } from "vitest";
+import { render, cleanup } from "@testing-library/react";
+import { StatRow } from "./StatRow.tsx";
+
+afterEach(cleanup);
+
+describe("StatRow", () => {
+  it("renders nothing for an empty item list", () => {
+    const { container } = render(<StatRow items={[]} />);
+    expect(container.firstChild).toBeNull();
+  });
+
+  it("renders each item's value and label, in order", () => {
+    const { container } = render(
+      <StatRow
+        items={[
+          { label: "Distance", value: "30.0 km" },
+          { label: "Time", value: "1h 0m" },
+          { label: "Avg speed", value: "30.0 km/h" },
+        ]}
+      />,
+    );
+    const labels = [...container.querySelectorAll("dt")].map((el) => el.textContent);
+    const values = [...container.querySelectorAll("dd")].map((el) => el.textContent);
+    expect(labels).toEqual(["Distance", "Time", "Avg speed"]);
+    expect(values).toEqual(["30.0 km", "1h 0m", "30.0 km/h"]);
+  });
+});