refactor(autosave): drop unused handleMentionsChange + getPendingMentions exports

Felix #2: both were exported anticipating a future use that never came —
the editor only emits text+mentions through handleTextChange. Dead public
surface invites stale code; ship the smaller API.

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

frontend/src/lib/hooks/useBlockAutoSave.svelte.ts

@@ -23,10 +23,6 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 		saveStates.set(blockId, state);
 	}
 
-	function getPendingMentions(blockId: string, fallback: PersonMention[]): PersonMention[] {
-		return pendingMentions.get(blockId) ?? fallback;
-	}
-
 	async function executeSave(blockId: string): Promise<void> {
 		const text = pendingTexts.get(blockId);
 		if (text === undefined) return;
@@ -90,12 +86,6 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 		scheduleDebounce(blockId);
 	}
 
-	function handleMentionsChange(blockId: string, mentionedPersons: PersonMention[]): void {
-		pendingMentions.set(blockId, mentionedPersons);
-		// Mentions changes always accompany text changes from the editor, so the
-		// text-debounce timer covers them too.
-	}
-
 	function handleBlur(): void {
 		for (const [blockId] of [...debounceTimers]) {
 			clearDebounce(blockId);
@@ -150,9 +140,7 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 
 	return {
 		getSaveState,
-		getPendingMentions,
 		handleTextChange,
-		handleMentionsChange,
 		handleBlur,
 		handleRetry,
 		clearBlock,