feat: 2026.1.0-alpha.16 – dashboard banner refactor, period auto-activation, text & docs

Dashboard (dashboard/src/dashboard.tsx, settings.tsx, apiAcademicPeriods.ts):
- Refactor loadHolidayStatus to useCallback with stable empty-deps reference;
  removes location.pathname dependency that caused overlapping API calls at mount
  and left the banner unresolved via request-sequence cancellation
- Add key prop derived from severity:text to Syncfusion MessageComponent to force
  remount on state change, fixing stale banner that ignored React prop/children updates
- Correct German transliterated text to proper Umlauts throughout visible UI strings
  (fuer -> für, oe -> ö, ae -> ä etc. across dashboard and settings views)

Backend (server/init_academic_periods.py):
- Refactor to idempotent two-phase flow: seed default periods only when table is
  empty; on every run activate exactly the non-archived period covering date.today()
- Enforces single-active invariant by deactivating all periods before promoting match
- Emits explicit warning when no period covers current date instead of doing nothing

Deployment (docker-compose.prod.yml):
- Add init_academic_periods.py to server startup chain after migrations and defaults;
  eliminates manual post-deploy step to set an active academic period

Release docs:
- program-info.json: bump to 2026.1.0-alpha.16; fix JSON parse error caused by
  typographic curly quotes in the new changelog entry
- TECH-CHANGELOG.md: detailed alpha.16 section with root-cause motivation for both
  dashboard refactoring decisions (unstable callback ref + Syncfusion stale render)
- DEV-CHANGELOG.md: document dashboard refactor, Syncfusion key fix, Umlaut changes,
  and program-info JSON regression and fix
- README.md: add Latest Release Highlights section for alpha.16
- .github/copilot-instructions.md: sync file map, prod bootstrap note, backend and
  frontend pattern additions for academic period init and Syncfusion remount pattern

dashboard/public/program-info.json

@@ -1,6 +1,6 @@
 {
   "appName": "Infoscreen-Management",
-  "version": "2026.1.0-alpha.15",
+  "version": "2026.1.0-alpha.16",
   "copyright": "© 2026 Third-Age-Applications",
   "supportContact": "support@third-age-applications.com",
   "description": "Eine zentrale Verwaltungsoberfläche für digitale Informationsbildschirme.",
@@ -30,6 +30,16 @@
     "commitId": "9f2ae8b44c3a"
   },
   "changelog": [
+    {
+      "version": "2026.1.0-alpha.16",
+      "date": "2026-04-02",
+      "changes": [
+        "✅ Dashboard: Der Ferienstatus-Banner zeigt die aktive akademische Periode jetzt zuverlässig nach Hard-Refresh und beim Wechsel zwischen Dashboard und Einstellungen.",
+        "🧭 Navigation: Der Link vom Ferienstatus-Banner zu den Einstellungen bleibt stabil und funktioniert konsistent für Admin-Rollen.",
+        "🚀 Deployment: Akademische Perioden werden nach Initialisierung automatisch für das aktuelle Datum aktiviert (kein manueller Aktivierungsschritt direkt nach Rollout mehr nötig).",
+        "🔤 Sprache: Mehrere deutsche UI-Texte im Dashboard wurden auf korrekte Umlaute umgestellt (zum Beispiel für, prüfen, Vorfälle und Ausfälle)."
+      ]
+    },
     {
       "version": "2026.1.0-alpha.15",
       "date": "2026-03-31",