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/e2e/documents.spec.ts

@@ -383,12 +383,16 @@ test.describe('PDF annotations — admin', () => {
 		// Record count now — the draw test may have created more than one annotation
 		const countBefore = await page.locator('[data-testid^="annotation-"]').count();
 
-		// Enable annotate mode to show delete buttons
+		// Enable annotate mode — deletion is only available while annotating
 		await page.getByRole('button', { name: /^annotieren$/i }).click();
 
-		const deleteBtn = page.getByRole('button', { name: /annotation löschen/i }).first();
-		await expect(deleteBtn).toBeVisible({ timeout: 8000 });
-		await deleteBtn.click();
+		// The on-canvas delete button was removed (issue #722). Delete via the
+		// kept keyboard shortcut: focus an annotation, press Delete, confirm.
+		const annotation = page.locator('[data-testid^="annotation-"]').first();
+		await annotation.click();
+		await annotation.press('Delete');
+
+		await page.getByRole('button', { name: /^bestätigen$/i }).click();
 
 		await expect(page.locator('[data-testid^="annotation-"]')).toHaveCount(countBefore - 1, {
 			timeout: 8000