feat(oxibadge): warn before losing unexported review edits, add undo

The review screen now tracks whether there are edits not yet exported
and asks for confirmation before discarding them: clicking back, or
closing the app window entirely (intercepted via onCloseRequested,
which needed a core:window:allow-destroy capability that wasn't
granted by core:default). The confirmation dialog (ConfirmDialog.svelte)
is generic, built on a shared ModalFrame extracted from ExportModal so
the two don't duplicate the backdrop/box styling.

Also adds undo: a per-row button (icon-only, left of the note, shown
when that day has edits) reverts just that day instantly, and a
per-employee "undo all" (icon in a red box, last among the header's
tools) reverts every day for the current employee after a confirmation
dialog, since that one can't be undone itself.

ActionButton moves out of review/ next to IconButton/PillButton, since
ConfirmDialog (outside the review feature) now uses it too.

Assisted-by: Claude Sonnet 5 <noreply@anthropic.com>

oxibadge/src/lib/components/settings/ReaderCard.svelte

@@ -7,7 +7,7 @@
   import { PRIMARY_READER_INDEX, type EmployeeCache, type EmployeeNames } from '$lib/employees';
   import Field from '../Field.svelte';
   import ErrorBanner from '../ErrorBanner.svelte';
-  import ActionButton from '../review/ActionButton.svelte';
+  import ActionButton from '../ActionButton.svelte';
   import PillButton from '../PillButton.svelte';
   import EmployeesDrawer from './EmployeesDrawer.svelte';
   import { IPV4 } from './logic';