feat(transcription): swap plain textarea for PersonMentionEditor and thread mentionedPersons through autosave

- TranscriptionBlockData now carries mentionedPersons (matches backend
  schema added in PR-A).
- useBlockAutoSave.saveFn signature widens to (blockId, text, mentions);
  pendingMentions is tracked alongside pendingTexts and is preserved on
  failure so a retry resends the in-flight payload (B12).
- TranscriptionBlock.svelte renders <PersonMentionEditor>, exposing the
  textarea node back through a captureTextarea callback so the existing
  quote-selection feature still works.
- saveBlock in routes/documents/[id]/+page.svelte forwards mentions on
  PUT.
- flushOnUnload sends mentions in the keepalive payload too.

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

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -16,6 +16,10 @@ type Props = {
 	disabled?: boolean;
 	onfocus?: () => void;
 	onblur?: () => void;
+	// Optional escape hatch: lets the parent observe the underlying textarea node
+	// (e.g. to read selection bounds for quote-selection features). Returning a
+	// cleanup function from the parent is not required.
+	captureTextarea?: (node: HTMLTextAreaElement) => void | (() => void);
 };
 
 let {
@@ -25,7 +29,8 @@ let {
 	rows = 1,
 	disabled = false,
 	onfocus,
-	onblur
+	onblur,
+	captureTextarea
 }: Props = $props();
 
 let query: string | null = $state(null);
@@ -38,7 +43,9 @@ let debounceTimer: ReturnType<typeof setTimeout> | undefined;
 
 function attachTextarea(node: HTMLTextAreaElement) {
 	textarea = node;
+	const parentCleanup = captureTextarea?.(node);
 	return () => {
+		parentCleanup?.();
 		textarea = null;
 	};
 }