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>

packages/i18n/src/locales/de.ts

@@ -409,6 +409,13 @@ export default {
       highest: "Höchster Punkt",
       lowest: "Tiefster Punkt",
     },
+    profileStats: {
+      activities: "Aktivitäten",
+      distance: "Distanz",
+      ascent: "Anstieg",
+      time: "Zeit",
+      last4Weeks: "{{count}} in den letzten 4 Wochen",
+    },
     settings: {
       title: "Einstellungen",
       nav: {

packages/i18n/src/locales/en.ts

@@ -409,6 +409,13 @@ export default {
       highest: "Highest",
       lowest: "Lowest",
     },
+    profileStats: {
+      activities: "Activities",
+      distance: "Distance",
+      ascent: "Ascent",
+      time: "Time",
+      last4Weeks: "{{count}} in the last 4 weeks",
+    },
     settings: {
       title: "Settings",
       nav: {