allow ~ to be used in config

following config throws error

```
    "filesystem": {
      "command": "npx",
      "args": [
        "-y",
        "@modelcontextprotocol/server-filesystem",
        "~"
      ]
    },
```

```
Error accessing directory ~: Error: ENOENT: no such file or directory, stat '~'
    at async Object.stat (node:internal/fs/promises:1032:18)
    at async file:///Users/USER_NAME/.npm/_npx/a3241bba59c344f5/node_modules/@modelcontextprotocol/server-filesystem/dist/index.js:33:23━━━━━━━━━━━━━━━━━
    at async Promise.all (index 0)
    at async file:///Users/USER_NAME/.npm/_npx/a3241bba59c344f5/node_modules/@modelcontextprotocol/server-filesystem/dist/index.js:31:1 {
  errno: -2,
  code: 'ENOENT',
  syscall: 'stat',
  path: '~'
}
```

this commit fixes error and allows to set ~ as allowed directory

src/filesystem/index.ts

@@ -42,7 +42,7 @@ const allowedDirectories = args.map(dir =>
 // Validate that all directories exist and are accessible
 await Promise.all(args.map(async (dir) => {
   try {
-    const stats = await fs.stat(dir);
+    const stats = await fs.stat(expandHome(dir));
     if (!stats.isDirectory()) {
       console.error(`Error: ${dir} is not a directory`);
       process.exit(1);