Let a day removed from a period stay removed

Taking one day out of a period the planner placed did nothing that
lasted: the period still asked for a full week, so the next sync put the
day straight back. The option was disabled rather than lying about it.

A removed day is now recorded as its own thing — no leave for this
parent, this child, this date — and the planner honours it on every
later sync. It is kept apart from the working-day overrides, which
answer a different question about the calendar rather than about the
plan.

The day belongs to the parent whose band was clicked, so removing one
parent's day leaves their partner's alone; where both parents were to be
off on one funding arrangement, the partner is simply funded on their
own instead. A removed day shortens what the period is asking for rather
than counting as a day the plan failed to find, so the advice about
closing a shortfall does not start arguing for a day nobody wants.

Nothing is removed without a way back: the toast undoes it, and the day
itself is marked in the calendar and offers to restore.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ei7gCZzdd751uZtPzb1Pep

frontend/tests/fixtures/mockData.ts

@@ -19,6 +19,7 @@ import type {
   Tip,
   VacationDays,
   Workspace,
+  PlannedDayExclusion,
 } from '@/types'
 
 // ---------------------------------------------------------------------------
@@ -195,6 +196,21 @@ export function makeDayOverride(overrides: Partial<DayOverride> = {}): DayOverri
   }
 }
 
+export function makePlannedDayExclusion(
+  overrides: Partial<PlannedDayExclusion> = {},
+): PlannedDayExclusion {
+  return {
+    id: 'ex-1',
+    workspaceId: 'ws-1',
+    childId: 'child-1',
+    parentPersonalDataUuid: 'parent-1',
+    date: '2027-02-03',
+    createdAt: '2027-01-01T00:00:00',
+    updatedAt: '2027-01-01T00:00:00',
+    ...overrides,
+  }
+}
+
 // ---------------------------------------------------------------------------
 // Tips
 // ---------------------------------------------------------------------------