fix(desktop): don't let uiohook-napi rebuild abort the whole workspace install

The @backspace/desktop postinstall runs `electron-rebuild -f -w uiohook-napi`,
which compiles a native module from source and needs a C++ toolchain
(make/g++/python3). On a bare host without those — a typical VPS a self-hoster
clones onto — that failure took the ENTIRE `pnpm install` down (exit 255), so the
server never installed either, even though uiohook-napi is desktop-only and
irrelevant to hosting.

Make the rebuild non-fatal: on failure it now prints a clear one-line warning and
continues. Machines with build tools (CI, desktop devs) are byte-identical — the
fallback never fires; only toolless boxes (which aren't building the desktop app
anyway) skip it. Verified on a bare x86 box: full `pnpm install` now exits 0 and
the server builds and boots from source. Docs note the build-tool requirement for
desktop work and point self-hosters at the Docker installer.

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"
+    "postinstall": "electron-rebuild -f -w uiohook-napi || 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-updater": "^6.3.0",