feat(timeline): add EventForm curator create/edit form

One component for both routes: /new renders it empty, /[id]/edit seeds it from a
TimelineEventView. Composes EventTypeSelect, the shared DatePrecisionField, a
plain-textarea description, PersonMultiSelect and DocumentMultiSelect (personIds
/documentIds hidden inputs). lg:grid-cols-[2fr_1fr] collapsing to one column
below lg, sticky save bar, beforeNavigate unsaved-changes guard, submitting flag
via use:enhance (disabled submit), and a delete form gated by getConfirmService()
read lazily so the component mounts cleanly in isolation. Title/description/chip
labels render via default {...} escaping (CWE-79). Seeded DocumentRefs degrade
gracefully to DocumentOption (no precision fields). Pickers gain an inputId prop
so <label for> associates the control; eslint boundaries now lets timeline import
person+document (mirrors the geschichte editor). 6/6 component specs green.

Refs #781
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/DocumentMultiSelect.svelte

@@ -13,13 +13,16 @@ interface Props {
 	hiddenInputName?: string;
 	/** Empty-state text shown inside the chip container when nothing is selected. */
 	emptyLabel?: string;
+	/** id of the search input so a <label for=...> can be associated. */
+	inputId?: string;
 }
 
 let {
 	selectedDocuments = $bindable([]),
 	placeholder = m.geschichte_editor_search_document(),
 	hiddenInputName = 'documentIds',
-	emptyLabel = undefined
+	emptyLabel = undefined,
+	inputId = undefined
 }: Props = $props();
 
 let searchTerm = $state('');
@@ -97,6 +100,7 @@ function removeDocument(id: string | undefined) {
 
 		<input
 			bind:this={inputEl}
+			id={inputId}
 			type="text"
 			autocomplete="off"
 			bind:value={searchTerm}