fix(extension): generate the API token with a CSPRNG and restrict the discovery file

The bearer token came from Math.random(), which is predictable. Use crypto.randomBytes
and write piagentui-port.json (which stores the token) with mode 0600.

extensions/piagentui-server.ts

@@ -3,7 +3,7 @@ import { WebSocketServer, WebSocket } from 'ws'
 import * as http from 'node:http'
 import * as fs from 'node:fs'
 import * as path from 'node:path'
-import { randomUUID } from 'node:crypto'
+import { randomBytes, randomUUID } from 'node:crypto'
 import {
   validateBearerToken,
   preventPathTraversal,
@@ -721,7 +721,8 @@ export default function (pi: ExtensionAPI) {
   }
 
   function generateBearerToken(): string {
-    return Array.from({ length: 4 }, () => Math.random().toString(36).substring(2)).join('-')
+    // CSPRNG: the token is the only credential guarding the local API.
+    return randomBytes(24).toString('base64url')
   }
 
   const settingsStore = createPiAgentUiSettingsStore(resolvePiAgentUiSettingsPaths())
@@ -2065,7 +2066,12 @@ export default function (pi: ExtensionAPI) {
           if (!fs.existsSync(piAgentDir)) {
             fs.mkdirSync(piAgentDir, { recursive: true })
           }
-          fs.writeFileSync(discoveryPath, JSON.stringify({ port: localPort, token: localToken }, null, 2), 'utf8')
+          // The file holds the API token: keep it readable by the owner only (mode is ignored on Windows).
+          fs.writeFileSync(discoveryPath, JSON.stringify({ port: localPort, token: localToken }, null, 2), {
+            encoding: 'utf8',
+            mode: 0o600,
+          })
+          fs.chmodSync(discoveryPath, 0o600)
           console.log(`[PiAgentUi] Wrote connection info to: ${discoveryPath}`)
         } catch (err) {
           console.error('[PiAgentUi] Failed to write discovery file:', err)