feat(helpdesk): redesign Historical tab with per-agent performance widgets

Each agent gets a card with three insight columns:
- Resolution time: Ø / Min / Max (formatted as hours or days)
- Top 3 customers by resolved ticket count (ranked list)
- Top 3 categories by resolved ticket count (ranked list)

Backend collects resolved ticket details per agent: customer names,
categories, and resolution hours. Aggregates top-3 and min/max/avg.
Replaces the old compact single-row layout with rich widget cards
matching the Current tab pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/i18n/default_de.json

@@ -296,6 +296,8 @@
   "Not assigned": "Nicht zugewiesen",
   "Could not load team dashboard.": "Team-Dashboard konnte nicht geladen werden.",
   "No team data available.": "Keine Team-Daten verfügbar.",
+  "Top customers": "Top-Kunden",
+  "Top categories": "Top-Kategorien",
   "Current": "Aktuell",
   "Performance": "Leistung",
   "Historical": "Historisch",