Let the calendar jump to any month or year, in both directions

Clicking the month or year label only ever stepped forward
(`month + 1`, `year + 1`), so the only way back was the prev-month arrow,
one month at a time. Jumping back a year was impossible.

Both labels are now selects, so any month or year is one click away in
either direction. Selects rather than +/- steppers because the app
already uses them elsewhere, and because stepping in a second direction
would still not give the "quickly jump" part of the original intent.

Year options are bounded to 2026-2055, the calendar cache range: outside
it the backend raises UncachedYearError and returns 422, so offering
those years would hand the user an error instead of a calendar.

The prev/next month arrows are unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RXMhc9nT6sNz1SL4j8dznL

frontend/src/i18n/en/calendar.ts

@@ -1,6 +1,8 @@
 export const calendar = {
   'calendar.advanceMonthAria': '{month} - click to advance month',
   'calendar.advanceYearAria': '{year} - click to advance year',
+  'calendar.selectMonthAria': 'Select month',
+  'calendar.selectYearAria': 'Select year',
   'calendar.deleteError': 'Failed to delete the planned day',
   'calendar.legendDoubleDay': 'Double day',
   'calendar.legendIncomeBased': 'Income-based rate (sjukpenningnivå)',