feat: period-scoped holiday management, archive lifecycle, and docs/release sync

- add period-scoped holiday architecture end-to-end
	- model: scope `SchoolHoliday` to `academic_period_id`
	- migrations: add holiday-period scoping, academic-period archive lifecycle, and merge migration head
	- API: extend holidays with manual CRUD, period validation, duplicate prevention, and overlap merge/conflict handling
	- recurrence: regenerate holiday exceptions using period-scoped holiday sets

- improve frontend settings and holiday workflows
	- bind holiday import/list/manual CRUD to selected academic period
	- show detailed import outcomes (inserted/updated/merged/skipped/conflicts)
	- fix file-picker UX (visible selected filename)
	- align settings controls/dialogs with defined frontend design rules
	- scope appointments/dashboard holiday loading to active period
	- add shared date formatting utility

- strengthen academic period lifecycle handling
	- add archive/restore/delete flow and backend validations/blocker checks
	- extend API client support for lifecycle operations

- release/docs updates and cleanup
	- bump user-facing version to `2026.1.0-alpha.15` with new changelog entry
	- add tech changelog entry for alpha.15 backend changes
	- refactor README to concise index and archive historical implementation docs
	- fix Copilot instruction link diagnostics via local `.github` design-rules reference

dashboard/src/dateFormatting.ts

@@ -0,0 +1,15 @@
+export function formatIsoDateForDisplay(isoDate: string | null | undefined): string {
+  if (!isoDate) {
+    return '-';
+  }
+
+  try {
+    const parsed = new Date(`${isoDate}T00:00:00`);
+    if (Number.isNaN(parsed.getTime())) {
+      return isoDate;
+    }
+    return parsed.toLocaleDateString('de-DE');
+  } catch {
+    return isoDate;
+  }
+}