Decouple Desktop UI into monorepo app (#5912)

## Summary

Extracts desktop UI into apps/desktop-ui package with minimal changes.

## Changes

- **What**:
- Separates desktop-specific code into standalone package with
independent Vite config, router, and i18n
- Drastically simplifies the main app router by removing all desktop
routes
  - Adds a some code duplication, most due to the existing design
- Some duplication can be refactored to be *simpler* on either side - no
need to split things by `isElectron()`
  - Rudimentary storybook support has been added
- **Breaking**: Stacked PR for publishing must be merged before this PR
makes it to stable core (but publishing _could_ be done manually)
  - #5915
- **Dependencies**: Takes full advantage of pnpm catalog. No additional
dependencies added.

## Review Focus

- Should be no changes to normal frontend operation
- Scripts added to root package.json are acceptable
- The duplication in this PR is copied as is, wherever possible. Any
corrections or fix-ups beyond the scope of simply migrating the
functionality as-is, can be addressed in later PRs. That said, if any
changes are made, it instantly becomes more difficult to separate the
duplicated code out into a shared utility.
  - Tracking issue to address concerns: #5925

### i18n

Fixing i18n is out of scope for this PR. It is a larger task that we
should consider carefully and implement properly. Attempting to isolate
the desktop i18n and duplicate the _current_ localisation scripts would
be wasted energy.

apps/desktop-ui/src/components/install/GpuPicker.vue

@@ -0,0 +1,103 @@
+<template>
+  <div
+    class="grid grid-rows-[1fr_auto_auto_1fr] w-full max-w-3xl mx-auto h-[40rem] select-none"
+  >
+    <h2 class="font-inter font-bold text-3xl text-neutral-100 text-center">
+      {{ $t('install.gpuPicker.title') }}
+    </h2>
+
+    <!-- GPU Selection buttons - takes up remaining space and centers content -->
+    <div class="flex-1 flex gap-8 justify-center items-center">
+      <!-- Apple Metal / NVIDIA -->
+      <HardwareOption
+        v-if="platform === 'darwin'"
+        :image-path="'/assets/images/apple-mps-logo.png'"
+        placeholder-text="Apple Metal"
+        subtitle="Apple Metal"
+        :value="'mps'"
+        :selected="selected === 'mps'"
+        :recommended="true"
+        @click="pickGpu('mps')"
+      />
+      <HardwareOption
+        v-else
+        :image-path="'/assets/images/nvidia-logo-square.jpg'"
+        placeholder-text="NVIDIA"
+        :subtitle="$t('install.gpuPicker.nvidiaSubtitle')"
+        :value="'nvidia'"
+        :selected="selected === 'nvidia'"
+        :recommended="true"
+        @click="pickGpu('nvidia')"
+      />
+      <!-- CPU -->
+      <HardwareOption
+        placeholder-text="CPU"
+        :subtitle="$t('install.gpuPicker.cpuSubtitle')"
+        :value="'cpu'"
+        :selected="selected === 'cpu'"
+        @click="pickGpu('cpu')"
+      />
+      <!-- Manual Install -->
+      <HardwareOption
+        placeholder-text="Manual Install"
+        :subtitle="$t('install.gpuPicker.manualSubtitle')"
+        :value="'unsupported'"
+        :selected="selected === 'unsupported'"
+        @click="pickGpu('unsupported')"
+      />
+    </div>
+
+    <div class="pt-12 px-24 h-16">
+      <div v-show="showRecommendedBadge" class="flex items-center gap-2">
+        <Tag
+          :value="$t('install.gpuPicker.recommended')"
+          class="bg-neutral-300 text-neutral-900 rounded-full text-sm font-bold px-2 py-[1px]"
+        />
+        <i-lucide:badge-check class="text-neutral-300 text-lg" />
+      </div>
+    </div>
+
+    <div class="text-neutral-300 px-24">
+      <p v-show="descriptionText" class="leading-relaxed">
+        {{ descriptionText }}
+      </p>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import type { TorchDeviceType } from '@comfyorg/comfyui-electron-types'
+import Tag from 'primevue/tag'
+import { computed } from 'vue'
+
+import HardwareOption from '@/components/install/HardwareOption.vue'
+import { st } from '@/i18n'
+import { electronAPI } from '@/utils/envUtil'
+
+const selected = defineModel<TorchDeviceType | null>('device', {
+  required: true
+})
+
+const electron = electronAPI()
+const platform = electron.getPlatform()
+
+const showRecommendedBadge = computed(
+  () => selected.value === 'mps' || selected.value === 'nvidia'
+)
+
+const descriptionKeys = {
+  mps: 'appleMetal',
+  nvidia: 'nvidia',
+  cpu: 'cpu',
+  unsupported: 'manual'
+} as const
+
+const descriptionText = computed(() => {
+  const key = selected.value ? descriptionKeys[selected.value] : undefined
+  return st(`install.gpuPicker.${key}Description`, '')
+})
+
+const pickGpu = (value: TorchDeviceType) => {
+  selected.value = value
+}
+</script>