fix(transcription): remove annotation canvas delete button that obscured text (#722)

The per-annotation delete button (a 44px circular control pinned to the
box's top-right) overlapped the box below and obscured the underlying
document text. It was redundant: every user-drawn annotation has a
transcription block, and the right-hand panel already offers a
non-overlapping delete per block that cascades to the annotation.

Remove the visible button and its `deleteVisible` derived. Keep the
keyboard Delete shortcut (and its `showDelete`/`onDeleteRequest`/
`deleteAnnotation` wiring) — it obscures nothing and remains a
power-user path and the only cleanup route for orphan annotations.

Tests: replace the button-render/click specs with contract tests
asserting no delete button ever renders; repoint the e2e delete flow
to the keyboard shortcut + confirm dialog.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/annotation/AnnotationLayer.svelte.spec.ts

@@ -98,23 +98,13 @@ describe('AnnotationLayer', () => {
 		expect(el2.style.opacity).toBe('1');
 	});
 
-	it('does not show delete button when annotation is not hovered or active', async () => {
+	// The on-canvas delete button was removed (issue #722). Even in annotate mode
+	// with an active annotation, no delete button must render over the document.
+	it('never renders a delete button, even in annotate mode with an active annotation', async () => {
 		render(AnnotationLayer, {
 			annotations: [makeAnnotation('ann-1')],
 			canDraw: true,
 			color: '#00C7B1',
-			onDraw: () => {}
-		});
-
-		await expect.element(page.getByTestId('annotation-ann-1')).toBeInTheDocument();
-		await expect.element(page.getByTestId('annotation-delete-ann-1')).not.toBeInTheDocument();
-	});
-
-	it('does not show delete button when canDraw is false even if annotation is active', async () => {
-		render(AnnotationLayer, {
-			annotations: [makeAnnotation('ann-1')],
-			canDraw: false,
-			color: '#00C7B1',
 			activeAnnotationId: 'ann-1',
 			onDraw: () => {}
 		});