feat(web): Uebersetzungsschluessel + Hilfsmodul calendar-month.ts fuer Kalender-Widget

- 16 neue Uebersetzungsschluessel unter widgets.calendar in de.json/en.json (Monatsnavigation, Naechste-Termine-Block, Einstellungsfelder)
- Neues Hilfsmodul calendar-month.ts: resolveCalendarConfig, buildCalendarDays, groupEventsByDate, computeFetchWindow, selectUpcomingEvents, Formatierungsfunktionen (quick-260916-htc)
- 8 Unit-Tests fuer das Hilfsmodul, Umlaut-Waechter weiterhin 3/3 gruen

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/web/src/messages/en.json

@@ -208,6 +208,22 @@
       "loading": "Loading...",
       "emptyNoSources": "No calendar sources configured",
       "emptyNoEvents": "No upcoming events",
+      "nothingSelected": "Nothing selected to display",
+      "monthPrev": "Back",
+      "monthNext": "Next",
+      "monthToday": "Back to the current month",
+      "upcomingTitle": "Upcoming events",
+      "tooltipMore": "More events available",
+      "allDay": "all day",
+      "configShowMonth": "Show month view",
+      "configMaxEvents": "Number of events",
+      "configMaxEventsNone": "Hide",
+      "configMaxEventsOne": "1 event",
+      "configMaxEventsMany": "{count} events",
+      "configLookahead": "Time range",
+      "configLookaheadOption": "Next {days} days",
+      "configSourcesHint": "Manage calendar sources under",
+      "configSourcesLink": "Settings → Dashboard → Calendar",
       "connectionSuccess": "Connection successful",
       "connectionError": "Connection failed. Please check the URL and credentials.",
       "addSource": "Add source",