fix: resolve lost updates in multi-user collaboration sync

Concurrent edits to shared notes (e.g., one user checks a shopping list
item while another adds an item) were lost because the sync system used
whole-document Last-Write-Wins. This replaces it with a 3-way line-level
content merge using node-diff3, so changes to different lines are
preserved from both users.

Key changes:
- Add node-diff3 for 3-way line-level content merging
- Remove the timestamp gate in processSyncPush that rejected entire
  offline sync pushes when another user had updated the note
- Add content merge to both the sync push path and the PATCH endpoint,
  using version snapshots as the merge base
- Send baseVersion from the client so the server can find the correct
  common ancestor for 3-way merge
- Protect pending local fields during client pull merge by reading
  the sync queue and preserving queued-but-unpushed changes
- Add comprehensive tests for content merge, server sync, and CRDT merge

https://claude.ai/code/session_01FNpVBBjM6vKzfR8y9NM9Ci

tests/e2e/sharing.spec.ts

@@ -124,6 +124,37 @@ test.describe('Note Sharing', () => {
 		expect(syncedNote.content).toBe('Owner sync update');
 	});
 
+	test('Scenario: Both users\' edits to different fields are preserved after concurrent PATCH', async ({
+		authenticatedPage: page,
+		collabPage
+	}) => {
+		// Given a shared checklist note exists
+		const note = await createSharedNote(
+			page,
+			collabPage,
+			'Shopping List',
+			'- [ ] Milk\n- [ ] Bread\n- [ ] Eggs'
+		);
+
+		// When the owner checks Milk
+		await page.request.patch(`/api/notes/${note.id}`, {
+			data: { content: '- [x] Milk\n- [ ] Bread\n- [ ] Eggs', baseVersion: note.version }
+		});
+
+		// And the collaborator adds Butter (based on the same original version)
+		await collabPage.request.patch(`/api/notes/${note.id}`, {
+			data: { content: '- [ ] Milk\n- [ ] Bread\n- [ ] Eggs\n- [ ] Butter', baseVersion: note.version }
+		});
+
+		// Then both changes are preserved via content merge
+		const res = await page.request.get(`/api/notes/${note.id}`);
+		const merged = await res.json();
+		expect(merged.content).toContain('- [x] Milk');
+		expect(merged.content).toContain('- [ ] Butter');
+		expect(merged.content).toContain('- [ ] Bread');
+		expect(merged.content).toContain('- [ ] Eggs');
+	});
+
 	test('Scenario: Collaborator sees leave action and can leave a shared note', async ({
 		authenticatedPage: page,
 		collabPage