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

FRONTEND_DESIGN_RULES.md

@@ -38,7 +38,7 @@ Do not introduce Tailwind utility classes — Tailwind has been removed from the
 | Paginated list | `PagerComponent` | When a full grid is too heavy; default page size 5 or 10 |
 | Text input | `TextBoxComponent` | Use `cssClass="e-outline"` on form-heavy sections |
 | Numeric input | `NumericTextBoxComponent` | Always set `min`, `max`, `step`, `format` |
-| Single select | `DropDownListComponent` | Always set `fields={{ text, value }}` |
+| Single select | `DropDownListComponent` | Always set `fields={{ text, value }}`; do **not** add `cssClass="e-outline"` — only `TextBoxComponent` uses outline style |
 | Boolean toggle | `CheckBoxComponent` | Use `label` prop, handle via `change` callback |
 | Buttons | `ButtonComponent` | See section 4 |
 | Modal dialogs | `DialogComponent` | `isModal={true}`, `showCloseIcon={true}`, footer with Cancel + primary |