Keep "both parents" wishes both-off, and warn when they can't be

A lower-priority 'both' wish (e.g. a yearly autumn week) quietly fell back
to one parent per day once summer/Jul had used both parents' vacation, and
still counted as funded, so nothing warned.

- Allocator rescue pass: frees a vacation day by switching one parent's
  vacation to FP on another date (interior v+v, then Mon/Fri bridge days
  without creating an FP weekend wrap, then solo days) and spends it on the
  one-parent date as co-leave. Pinned days count as present.
- WindowOutcome.unmet_both_dates + new both_parents_unmet plan warning.
- Frontend maps the new fields; Dashboard warning card, year-row flag and a
  Calendar save-toast line list the affected years (sv + en).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/tests/fixtures/plan.ts

@@ -63,6 +63,9 @@ export function makeWindowOutcome(overrides: Partial<WindowOutcome> = {}): Windo
     weeksFunded: 6,
     shortfallDays: 0,
     strategy: 'parent_swap',
+    requestedStrategy: 'parent_swap',
+    strategyDegraded: false,
+    unmetBothDates: [],
     messageKey: 'planOutcomes.windowExplanation',
     data: {},
     costBreakdown: makeCostBreakdown({ free: 12, vacation: 19, sjukpenning: 11 }),