[Windows] Server never persists pointed element — hardcoded /tmp path breaks get-pointed-element #22

Summary

On Windows, get-pointed-element always returns "No element is currently pointed", even though the Chrome extension connects fine and reports ✓ Element sent successfully. The element never reaches the MCP tool.

Root cause

SharedStateService.SHARED_STATE_PATH is hardcoded to a POSIX path:

// packages/server/src/services/shared-state-service.ts:6
static SHARED_STATE_PATH = '/tmp/mcp-pointer-shared-state.json';

On Windows, Node resolves /tmp/... to C:\tmp\... (root of the current drive), which usually doesn't exist. So:

1. Extension sends the element → server receives it (📨 Received message from browser).
2. saveState() calls fs.writeFile('/tmp/...') → fails with ENOENT (directory doesn't exist).
3. The error is swallowed by the try/catch (only logger.error, and the Web Store build is IS_DEV=false, so nothing is visible in the SW console).
4. getPointedElement() → readState() → ENOENT → returns null → tool returns "No element is currently pointed".

The browser side is 100% fine — it's purely the server failing to persist state on Windows.

Environment

• OS: Windows 11
• Node: v24
• Chrome: 145
• Extension 0.6.0, @mcp-pointer/server@0.6.0
• Client: Claude Code

Reproduction

1. Install on a Windows machine that has no C:\tmp directory.
2. Alt+Click an element — the service worker console shows ✓ Element sent successfully.
3. Call get-pointed-element → always "No element is currently pointed".

Confirmation

Manually creating C:\tmp works around it: the state file appears at C:\tmp\mcp-pointer-shared-state.json and the tool returns the element. This confirms the path is the culprit.

Suggested fix

Use os.tmpdir() (cross-platform) instead of the hardcoded /tmp/:

import os from 'os';
import path from 'path';
static SHARED_STATE_PATH = path.join(os.tmpdir(), 'mcp-pointer-shared-state.json');

This preserves the cross-instance shared-state behavior (all instances for the same user resolve to the same temp dir) while working on Windows, macOS and Linux.

I have a PR ready for this and will link it shortly.