fix(ci): actually compile the native audio module, and prove it

The previous run went green and produced an installer with no system-audio
capture in it — the exact silent failure this module exists to prevent.

Two causes. electron-rebuild takes -w as one comma-separated list, not a
repeated flag; passing it twice made argv.w an array and the CLI threw
'argv.w.split is not a function', which also broke uiohook-napi's rebuild that
had been working. And pnpm 10 refuses to run a dependency's build script unless
it is listed in onlyBuiltDependencies, so node-gyp never ran for it at all —
the log said 'Ignored build scripts' and nothing else complained.

Neither surfaced because desktop's postinstall ends in , which
exists so contributors without build tools can install. That is reasonable
locally and dangerous in CI, so the workflow now asserts a compiled .node
exists and fails loudly when it does not, instead of trusting an exit code that
was designed to lie.

packages/desktop/package.json

@@ -18,7 +18,7 @@
     "clean": "rm -rf dist dist-electron",
     "test": "vitest run",
     "test:watch": "vitest",
-    "postinstall": "electron-rebuild -f -w uiohook-napi -w electron-native-screenshare || node -e \"console.warn('[desktop] uiohook-napi native rebuild skipped - needs build tools (make, g++, python3). Only required to RUN the desktop app; the server, web client, and Docker image are unaffected.')\""
+    "postinstall": "electron-rebuild -f -w uiohook-napi,electron-native-screenshare || node -e \"console.warn('[desktop] uiohook-napi native rebuild skipped - needs build tools (make, g++, python3). Only required to RUN the desktop app; the server, web client, and Docker image are unaffected.')\""
   },
   "dependencies": {
     "electron-native-screenshare": "^1.2.0",