feat(helpdesk): split team dashboard into Current and Performance tabs

Current tab: snapshot KPIs (open, unassigned, avg age) and per-agent
table with open/critical/age columns. No period filter needed.

Performance tab: period-filtered KPI (resolved) with segment control
(30/90/180/365d) and per-agent table sorted by resolved count.

Separates the two core questions: "Who needs help now?" vs.
"How did the team perform over time?"

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

modules/helpdesk/i18n/default_de.json

@@ -294,5 +294,7 @@
   "Resolved in period": "Gelöst im Zeitraum",
   "Not assigned": "Nicht zugewiesen",
   "Could not load team dashboard.": "Team-Dashboard konnte nicht geladen werden.",
-  "No team data available.": "Keine Team-Daten verfügbar."
+  "No team data available.": "Keine Team-Daten verfügbar.",
+  "Current": "Aktuell",
+  "Performance": "Leistung"
 }

modules/helpdesk/i18n/default_en.json

@@ -294,5 +294,7 @@
   "Resolved in period": "Resolved in period",
   "Not assigned": "Not assigned",
   "Could not load team dashboard.": "Could not load team dashboard.",
-  "No team data available.": "No team data available."
+  "No team data available.": "No team data available.",
+  "Current": "Current",
+  "Performance": "Performance"
 }