profile-stats: lifetime roll-up header on the profile

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

- getActivityStats(ownerId, { publicOnly }) in activities.server.ts: one indexed
  aggregate over stored columns (count, sum distance/ascent/elapsed duration) +
  a rolling last-4-weeks count. No cache table, no schema change, no GPX parsing
  (design §D1).
- Profile loader computes it scoped to the viewer (public-only for visitors,
  full totals for the owner); ProfileStats header renders count · distance ·
  ascent · time + "N in the last 4 weeks" via the shared StatRow + stats.ts
  formatters; hidden when there are no visible activities.
- i18n journal.profileStats.* in en + de.

Tests: ProfileStats component (jsdom: totals, empty, last-4-weeks toggle);
e2e asserts the owner roll-up counts their activities. typecheck + lint + unit
(journal 318) green; verified in the browser.

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

e2e/profile-stats.test.ts

@@ -0,0 +1,26 @@
+import { test, expect, gotoHydrated } from "./fixtures/test";
+import { setupVirtualAuthenticator, registerUser } from "./helpers/auth";
+
+test.describe("Profile stats", () => {
+  test("owner profile shows a roll-up header counting their activities", async ({ page }) => {
+    const cdp = await page.context().newCDPSession(page);
+    await setupVirtualAuthenticator(cdp);
+
+    const stamp = Date.now();
+    const username = `ps${stamp}`;
+    await registerUser(page, `ps-${stamp}@example.com`, username);
+
+    // Two activities → the owner's roll-up should count both.
+    for (const name of ["First outing", "Second outing"]) {
+      await gotoHydrated(page, "/activities/new");
+      await page.getByLabel("Name").fill(name);
+      await page.getByRole("button", { name: "Create Activity" }).click();
+      await expect(page).toHaveURL(/\/activities\/[0-9a-f-]+$/, { timeout: 10000 });
+    }
+
+    await gotoHydrated(page, `/users/${username}`);
+    // The roll-up header — the "N in the last 4 weeks" line is unique to it
+    // (the section headings below read "Activities (N)").
+    await expect(page.getByText("2 in the last 4 weeks")).toBeVisible();
+  });
+});