feat(transcription): clicking annotation focuses corresponding block

Pass activeAnnotationId to TranscriptionEditView. An $effect watches
it and sets activeBlockId to the block matching the annotation,
activating its turquoise focus border.

2 new tests (RED/GREEN):
- activates block matching activeAnnotationId (turquoise border)
- no block activated when activeAnnotationId is null

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -223,6 +223,7 @@ onMount(() => {
 					blocks={transcriptionBlocks}
 					canComment={canWrite}
 					currentUserId={currentUserId}
+					activeAnnotationId={activeAnnotationId}
 					onBlockFocus={handleBlockFocus}
 					onSaveBlock={saveBlock}
 					onDeleteBlock={deleteBlock}