feat(transcription): add drag-and-drop + arrow button reordering

TranscriptionBlock:
- Desktop: grip handle (⠿) on left side, serves as drag handle
- Mobile (<768px): ▲/▼ arrow buttons (44px tap targets) replace grip
- isFirst/isLast disable boundary arrows
- onMoveUp/onMoveDown callbacks for arrow button clicks

TranscriptionEditView:
- HTML5 drag-and-drop on block wrappers (only initiates from grip handle)
- Dragged block shows 40% opacity
- On drop: reorder array and call PUT /reorder endpoint
- Arrow handlers: swap adjacent blocks and call reorder endpoint

5 new tests:
- drag handle element present
- move-up disabled when isFirst
- move-down disabled when isLast
- onMoveUp fires on click
- onMoveDown fires on click

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

frontend/src/lib/components/TranscriptionBlock.svelte.spec.ts

@@ -121,3 +121,41 @@ describe('TranscriptionBlock — interactions', () => {
 		await expect.element(btn).toBeInTheDocument();
 	});
 });
+
+// ─── Reorder controls ────────────────────────────────────────────────────────
+
+describe('TranscriptionBlock — reorder controls', () => {
+	it('shows a drag handle element', async () => {
+		renderBlock();
+		const handle = document.querySelector('[data-drag-handle]');
+		expect(handle).not.toBeNull();
+	});
+
+	it('disables move-up button when isFirst', async () => {
+		renderBlock({ isFirst: true });
+		const btn = page.getByRole('button', { name: 'Nach oben' });
+		await expect.element(btn).toBeDisabled();
+	});
+
+	it('disables move-down button when isLast', async () => {
+		renderBlock({ isLast: true });
+		const btn = page.getByRole('button', { name: 'Nach unten' });
+		await expect.element(btn).toBeDisabled();
+	});
+
+	it('calls onMoveUp when up arrow clicked', async () => {
+		const onMoveUp = vi.fn();
+		renderBlock({ onMoveUp, isFirst: false });
+		const btn = page.getByRole('button', { name: 'Nach oben' });
+		await btn.click();
+		expect(onMoveUp).toHaveBeenCalled();
+	});
+
+	it('calls onMoveDown when down arrow clicked', async () => {
+		const onMoveDown = vi.fn();
+		renderBlock({ onMoveDown, isLast: false });
+		const btn = page.getByRole('button', { name: 'Nach unten' });
+		await btn.click();
+		expect(onMoveDown).toHaveBeenCalled();
+	});
+});