fix(document-picker): add aria-label to combobox input

The document search input had no accessible label — role="combobox"
without a label is an accessibility violation. Bound aria-label to
the existing placeholder prop so screen readers announce the field purpose.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/document/DocumentPickerDropdown.svelte

@@ -78,6 +78,7 @@ function formatDocLabel(doc: DocumentOption): string {
 		type="text"
 		role="combobox"
 		autocomplete="off"
+		aria-label={placeholder}
 		aria-expanded={picker.isOpen && picker.results.length > 0}
 		aria-controls={listboxId}
 		aria-autocomplete="list"