Tell the user when vacation days move between periods

reallocate_vacation_by_value swaps vacation onto higher-value dates 1-for-1
with parental-leave days. Add an Easter period with no vacation left and the
planner quietly takes vacation off the summer period, puts foraldrapenning
there, and spends it on the Easter klamdagar. Nothing said so.

sync_plan already emitted the evidence -- paired 'retyped' changes -- but
PlanChange.kind's 'retyped' existed only as a type declaration, rendered
nowhere, with no copy in either locale.

findVacationSwaps pairs a vacation -> FP retype with an FP -> vacation retype
for the same parent, aggregating multiple pairs into one group and ignoring
any retype that is not a genuine two-sided swap. summarizeVacationSwaps names
the periods via the existing resolveWishName, so a preset wish is
re-translated rather than shown raw, and falls back to dates when no single
wish resolves -- never a uuid. It joins the existing Calendar sync toast
rather than adding a second toast system.

Frontend 1356 tests, coverage 98.22 / 94.95 / 92.31 / 98.22, above all gates.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YZaX6xbuMEMXuyKA1fq2Mm

frontend/src/i18n/en/calendar.ts

@@ -88,6 +88,12 @@ export const calendar = {
     '{name} ({start}–{end}): only {daysFunded} of {daysWished} days funded — {shortfallDays} {shortfallDays|day|days} short',
   'calendar.toastRedDayShift':
     '{name}: moved to {start} – {end} to catch {days} extra free {days|day|days}',
+  // Reallocation pass — `reallocate_vacation_by_value` swaps vacation onto
+  // better dates 1-for-1 with parental leave. `{from}`/`{to}` already carry
+  // the period name plus its dates (e.g. "Summer (Jun 20, Jun 21)"), or the
+  // dates alone when no single period can be named — see vacationSwaps.ts.
+  'calendar.toastVacationSwap':
+    '{count} {count|vacation day|vacation days} moved from {from} to {to} — replaced with parental leave',
   'calendar.undoButton': 'Undo',
   'calendar.undoError': 'Failed to undo the change',
   'calendar.dismissToastAria': 'Dismiss the message',