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/src/styles/components.css

@@ -466,10 +466,15 @@
 }
 
 .year-row__wish-weeks--short,
-.year-row__shortfall {
+.year-row__shortfall,
+.year-row__both-unmet {
   color: var(--c-danger);
 }
 
+.year-row__both-unmet {
+  flex-basis: 100%;
+}
+
 .year-row__remedies {
   flex-basis: 100%;
   padding-left: var(--sp-4);