feat(projects): attach-event control in the cockpit

Search any event by name and attach it to the project (re-points
events.project_id via assignEvent). Lists the project's current events
above the search. en + de i18n. Completes event grouping UX — admins
can now regroup the auto-created per-event projects however they like.

frontend/src/i18n/locales/de.json

@@ -3410,6 +3410,13 @@
       "label": "Projekt",
       "none": "Kein Projekt"
     },
+    "events": {
+      "title": "Events",
+      "none": "Diesem Projekt sind noch keine Events zugeordnet.",
+      "searchPlaceholder": "Event zuordnen — nach Name suchen…",
+      "attached": "Event zugeordnet",
+      "attachFailed": "Event konnte nicht zugeordnet werden"
+    },
     "feed": {
       "title": "Aktivität",
       "empty": "Diesem Projekt ist noch nichts zugeordnet.",

frontend/src/i18n/locales/en.json

@@ -3410,6 +3410,13 @@
       "label": "Project",
       "none": "No project"
     },
+    "events": {
+      "title": "Events",
+      "none": "No events grouped under this project yet.",
+      "searchPlaceholder": "Attach an event — search by name…",
+      "attached": "Event attached",
+      "attachFailed": "Could not attach event"
+    },
     "feed": {
       "title": "Activity",
       "empty": "Nothing rolled up to this project yet.",