build(machine): add a web-only build:web target

The machine app's `build` script runs vue-tsc, the Vite build, two electron
tsc passes and an esbuild bundle. Serving the kiosk as a plain SPA needs only
the middle one, and the electron passes drag in native-addon typings that a
web build has no use for.

Add `build:web` (just `vite build`) with a turbo task that still builds the
workspace packages first via `dependsOn: ["^build"]`, so a consumer can run
`pnpm build:web` at the repo root and get `apps/machine/dist`.

`env: ["VITE_*"]` is declared on the task because the Vite vars are baked into
the bundle at build time — without it turbo would happily serve a cached
build produced under different env.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013A6683cCHnQxFUosx1krY4

turbo.json

@@ -5,6 +5,11 @@
       "dependsOn": ["^build"],
       "outputs": ["dist/**", ".next/**", "!.next/cache/**"]
     },
+    "build:web": {
+      "dependsOn": ["^build"],
+      "outputs": ["dist/**"],
+      "env": ["VITE_*"]
+    },
     "dev": {
       "cache": false,
       "persistent": true