Finish the redesign's edges: auth page, modals, dead code, docs

The sign-in page becomes a plain centred column with the brand mark
and a segmented login/register switch, modals lose their last inner
boxes, the red-days list collapses every year but the current one,
and the retired components' CSS, an unused list component and 23
unreferenced translation keys are removed. ARCHITECTURE.md documents
the Almanacka styling conventions and the per-year balance rule.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XBRLomdLK9jF7wU7a8AA6H

frontend/src/hooks/useLatestBalance.ts

@@ -27,10 +27,9 @@ interface UseLatestBalanceResult {
  * from either entry point.
  *
  * Imported directly from this file (not the `@/hooks` barrel) by both pages
- * — same escape hatch `VacationDaysList` uses for `useVacationDays` — so
- * their tests can drive it through a mocked `api.getParentalLeave` instead
- * of mocking the hook itself, while the rest of `@/hooks` stays blanket-
- * mocked.
+ * so their tests can drive it through a mocked `api.getParentalLeave`
+ * instead of mocking the hook itself, while the rest of `@/hooks` stays
+ * blanket-mocked.
  *
  * `personalDataUuid`/`childId` are null when no balance form is open — the
  * hook then no-ops and reports no saved balance, rather than fetching.