fix: build fails on Linux/Intel macOS due to missing native optional deps (#583)

* fix: declare platform-native optional deps so the frontend build resolves

The Linux (and macOS Intel) build of the frontend web bundle failed with:

  Cannot find module '../lightningcss.linux-x64-gnu.node'

The committed lockfile only carried native package nodes for darwin-arm64
(it was generated on Apple Silicon). On any other platform the required
native binaries for lightningcss and @tailwindcss/oxide have no resolution
entry in the lockfile, so npm silently skips them (they're optional),
breaking the Tailwind/webpack build in npm ci and npm install alike
(npm/cli#4828).

Declare the needed platform binaries as direct optionalDependencies so npm
records their nodes in the lockfile and installs them reliably on each
matching platform. Covers the platforms this repo actually builds on:
linux-x64-gnu (build_linux.sh / build_deb.sh, _build-linux.yml) and macOS
arm64/x64 (build_dmg.sh, _build-dmg.yml). Their os/cpu constraints mean each
installs only where it applies.

Verified: a clean `npm ci` now installs the Linux natives and the frontend
bundle builds end to end.

* chore: add changeset for Linux/Intel macOS native deps fix

Author: Davidnet

.changeset/fix-linux-build-native-deps.md

@@ -0,0 +1,5 @@
+---
+"kiji-privacy-proxy": patch
+---
+
+- Declared the platform-native optional dependencies so the frontend build resolves on Linux and Intel macOS, fixing builds that failed due to missing native optional deps.

package-lock.json

@@ -63,6 +63,14 @@
     "webpack-cli": "^7.0.3",
     "webpack-dev-server": "^5.2.5"
   },
+  "optionalDependencies": {
+    "@tailwindcss/oxide-darwin-arm64": "4.1.18",
+    "@tailwindcss/oxide-darwin-x64": "4.1.18",
+    "@tailwindcss/oxide-linux-x64-gnu": "4.1.18",
+    "lightningcss-darwin-arm64": "1.30.2",
+    "lightningcss-darwin-x64": "1.30.2",
+    "lightningcss-linux-x64-gnu": "1.30.2"
+  },
   "build": {
     "appId": "com.kiji.proxy",
     "productName": "Kiji Privacy Proxy",