refactor(TranscriptionBlock): migrate quote selection to Tiptap selectionUpdate (AC-7)

Replaces captureTextarea + handleTextareaMouseUp (which read selection
bounds off a real <textarea>) with an onSelectionChange callback prop
on PersonMentionEditor, wired to Tiptap's selectionUpdate event. The
editor emits the selected text directly so the parent no longer needs
DOM access.

Tests are updated to drive the contenteditable via the Selection API
instead of the now-deleted textarea.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/TranscriptionBlock.svelte

@@ -79,17 +79,6 @@ let leftBorderClass = $derived(
 	saveState === 'error' ? 'border-l-2 border-error' : active ? 'border-l-2 border-turquoise' : ''
 );
 
-// Single source of truth for the editor's textarea — stored on attach so
-// we can read selection bounds for quote selection without re-querying the DOM.
-let textareaEl: HTMLTextAreaElement | null = null;
-
-function captureTextarea(node: HTMLTextAreaElement) {
-	textareaEl = node;
-	return () => {
-		textareaEl = null;
-	};
-}
-
 function emitChange() {
 	onTextChange(localText, localMentions);
 }
@@ -101,17 +90,6 @@ async function handleDelete() {
 	});
 	if (confirmed) onDeleteClick();
 }
-
-function handleTextareaMouseUp() {
-	if (!textareaEl) return;
-	const start = textareaEl.selectionStart;
-	const end = textareaEl.selectionEnd;
-	if (start !== end) {
-		selectedQuote = localText.substring(start, end);
-	} else {
-		selectedQuote = null;
-	}
-}
 </script>
 
 <div
@@ -176,24 +154,22 @@ function handleTextareaMouseUp() {
 			{/if}
 		</div>
 
-		<!-- Textarea (now powered by PersonMentionEditor for @-mention typeahead) -->
-		<div onmouseup={handleTextareaMouseUp} role="presentation">
-			<PersonMentionEditor
-				bind:value={() => localText,
-					(v) => {
-						localText = v;
-						emitChange();
-					}}
-				bind:mentionedPersons={() => localMentions,
-					(next) => {
-						localMentions = next;
-						emitChange();
-					}}
-				placeholder={m.transcription_block_placeholder()}
-				onfocus={onFocus}
-				captureTextarea={captureTextarea}
-			/>
-		</div>
+		<!-- Editor powered by PersonMentionEditor (Tiptap) for @-mention typeahead -->
+		<PersonMentionEditor
+			bind:value={() => localText,
+				(v) => {
+					localText = v;
+					emitChange();
+				}}
+			bind:mentionedPersons={() => localMentions,
+				(next) => {
+					localMentions = next;
+					emitChange();
+				}}
+			placeholder={m.transcription_block_placeholder()}
+			onfocus={onFocus}
+			onSelectionChange={(text) => (selectedQuote = text)}
+		/>
 
 		{#if selectedQuote}
 			<p class="mt-1 text-xs text-ink-3">{m.transcription_block_quote_hint()}</p>