fix(server): use os.tmpdir() for shared state path (Windows compatibility)

[thiagomendonca-eu]

What

Fixes the hardcoded POSIX /tmp/ path used for the shared-state file, which silently breaks the tool on Windows.

Closes #22

Problem

// packages/server/src/services/shared-state-service.ts
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. As a result:

1. The extension sends the element and the server receives it (📨 Received message from browser).
2. saveState() → fs.writeFile('/tmp/...') fails with ENOENT.
3. The error is swallowed by the try/catch (only logger.error), so nothing is visible in the production build.
4. getPointedElement() → readState() → ENOENT → returns null.

Net effect: the extension reports ✓ Element sent successfully, but get-pointed-element always returns "No element is currently pointed" on Windows.

Fix

import os from 'os';
import path from 'path';

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

Cross-platform, and it preserves the cross-instance shared-state behavior — all server instances for the same user resolve to the same temp dir on every OS.

Testing

• Windows 11, Node 24, Chrome 145, extension/server 0.6.0, Claude Code.
• Before: no state file written; get-pointed-element always empty.
• Confirmed root cause: manually creating C:\tmp works around it (state file appears, tool returns the element).
• After (this fix): state file is written to os.tmpdir() and get-pointed-element returns the pointed element.
• No change on macOS/Linux: os.tmpdir() resolves to the existing system temp dir (/tmp on Linux, /var/folders/... on macOS).

Single-file change, no API or behavior change beyond the path resolution.