Add activity sort toggle on user profile page

Default sort is by activity date (startedAt); users can switch to
"Date added" (createdAt) via a URL query param (?sort=addedAt).
Activities without a startedAt fall to the bottom when sorted by date.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/i18n/src/locales/de.ts

@@ -351,6 +351,8 @@ export default {
       delete: "Aktivität löschen",
       deleteConfirm: "Möchtest du diese Aktivität wirklich löschen?",
       importedFrom: "Importiert von {{provider}}",
+      sortByDate: "Aktivitätsdatum",
+      sortByAdded: "Hinzugefügt am",
       visibility: {
         label: "Sichtbarkeit",
         private: "Privat",

packages/i18n/src/locales/en.ts

@@ -351,6 +351,8 @@ export default {
       delete: "Delete Activity",
       deleteConfirm: "Are you sure you want to delete this activity?",
       importedFrom: "Imported from {{provider}}",
+      sortByDate: "Activity date",
+      sortByAdded: "Date added",
       visibility: {
         label: "Visibility",
         private: "Private",