fix(timeline): track form dirtiness via change callbacks, not an $effect

The round-2 dirty-guard used an $effect that both read and wrote its `dirtyArmed`
$state, so the self-write re-triggered the effect and `dirty` flipped true on
mount — the beforeNavigate confirm then fired on every navigation away from an
untouched form (caught by the round-3 clean-agent review + the Svelte autofixer,
which flags assigning state inside $effect).

Replace it with the component's existing idiomatic pattern: DatePrecisionField,
PersonMultiSelect, and DocumentMultiSelect each gain an optional `onchange`
callback fired on a real user edit, and EventForm passes `markDirty` to all
three. Now date/precision/end-date and picker add/remove mark the form dirty
exactly like title/type/description already did — no effect, no mount-timing
trap. The new props are optional, so the other consumers (WhoWhenSection, the
document forms) are unaffected. Svelte autofixer: clean.

Addresses PR #832 review (round-3 clean-agent concern).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/DocumentMultiSelect.svelte

@@ -15,6 +15,8 @@ interface Props {
 	emptyLabel?: string;
 	/** id of the search input so a <label for=...> can be associated. */
 	inputId?: string;
+	/** Called when the selection changes (add/remove) — lets a parent track dirtiness. */
+	onchange?: () => void;
 }
 
 let {
@@ -22,7 +24,8 @@ let {
 	placeholder = m.geschichte_editor_search_document(),
 	hiddenInputName = 'documentIds',
 	emptyLabel = undefined,
-	inputId = undefined
+	inputId = undefined,
+	onchange = undefined
 }: Props = $props();
 
 let searchTerm = $state('');
@@ -54,10 +57,12 @@ function selectDocument(doc: DocumentOption) {
 	selectedDocuments = [...selectedDocuments, doc];
 	searchTerm = '';
 	picker.close();
+	onchange?.();
 }
 
 function removeDocument(id: string | undefined) {
 	selectedDocuments = selectedDocuments.filter((d) => d.id !== id);
+	onchange?.();
 }
 </script>