fix: use require.resolve to find hoisted node-pty in postinstall

When installed via npx, node-pty is hoisted as a sibling of
termbeam (not nested inside termbeam/node_modules/). The previous
postinstall used __dirname + 'node_modules/node-pty' which missed
the hoisted location. Use require.resolve('node-pty/package.json')
to find node-pty regardless of hoisting.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dorlugasigal

package.json

@@ -15,7 +15,7 @@
     "format": "prettier --write .",
     "lint": "node --check src/*.js bin/*.js",
     "prepublishOnly": "npm test",
-    "postinstall": "node -e \"try{const p=require('path'),fs=require('fs'),d=p.join(__dirname,'node_modules','node-pty','prebuilds');if(fs.existsSync(d)){for(const a of fs.readdirSync(d)){const s=p.join(d,a,'spawn-helper');try{fs.chmodSync(s,0o755)}catch{}}}}catch{}\""
+    "postinstall": "node -e \"try{const p=require('path'),fs=require('fs'),r=require.resolve('node-pty/package.json'),d=p.join(p.dirname(r),'prebuilds');if(fs.existsSync(d)){for(const a of fs.readdirSync(d)){const s=p.join(d,a,'spawn-helper');try{fs.chmodSync(s,0o755)}catch{}}}}catch{}\""
   },
   "keywords": [
     "terminal",