feat(document-picker): optional inputId prop for external label wiring (#795)

The input always carries an id (generated doc-picker-input-{uid} default,
mirroring the listbox id scheme). When a caller passes inputId it wires a
visible <label for> — the aria-label fallback is dropped then so the
visible label wins. JourneyAddBar stays untouched.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/lib/document/DocumentPickerDropdown.svelte

@@ -10,17 +10,21 @@ import {
 interface Props {
 	alreadyAddedIds?: Set<string>;
 	placeholder?: string;
+	/** Set when a visible <label for> is wired externally — replaces the aria-label fallback. */
+	inputId?: string;
 	onSelect: (doc: DocumentOption) => void;
 }
 
 let {
 	alreadyAddedIds = new Set(),
 	placeholder = m.journey_add_document(),
+	inputId = undefined,
 	onSelect
 }: Props = $props();
 
 const uid = $props.id();
 const listboxId = `doc-picker-listbox-${uid}`;
+const resolvedInputId = $derived(inputId ?? `doc-picker-input-${uid}`);
 
 const picker = createDocumentTypeahead();
 
@@ -82,7 +86,8 @@ function handleKeydown(e: KeyboardEvent) {
 		type="text"
 		role="combobox"
 		autocomplete="off"
-		aria-label={placeholder}
+		id={resolvedInputId}
+		aria-label={inputId ? undefined : placeholder}
 		aria-expanded={picker.isOpen}
 		aria-controls={picker.isOpen && !picker.loading && !picker.error && picker.results.length > 0
 			? listboxId