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/views/ManualConfigurationView.vue

@@ -0,0 +1,86 @@
+<template>
+  <BaseViewTemplate dark>
+    <!-- Installation Path Section -->
+    <div
+      class="comfy-installer grow flex flex-col gap-4 text-neutral-300 max-w-110"
+    >
+      <h2 class="text-2xl font-semibold text-neutral-100">
+        {{ $t('install.manualConfiguration.title') }}
+      </h2>
+
+      <p class="m-1 text-neutral-300">
+        <Tag
+          icon="pi pi-exclamation-triangle"
+          severity="warn"
+          :value="t('icon.exclamation-triangle')"
+        />
+        <strong class="ml-2">{{
+          $t('install.gpuSelection.customComfyNeedsPython')
+        }}</strong>
+      </p>
+
+      <div>
+        <p class="m-1 mb-4">
+          {{ $t('install.manualConfiguration.requirements') }}:
+        </p>
+        <ul class="m-0">
+          <li>{{ $t('install.gpuSelection.customManualVenv') }}</li>
+          <li>{{ $t('install.gpuSelection.customInstallRequirements') }}</li>
+        </ul>
+      </div>
+
+      <p class="m-1">{{ $t('install.manualConfiguration.createVenv') }}:</p>
+
+      <Panel :header="t('install.manualConfiguration.virtualEnvironmentPath')">
+        <span class="font-mono">{{ `${basePath}${sep}.venv${sep}` }}</span>
+      </Panel>
+
+      <p class="m-1">
+        {{ $t('install.manualConfiguration.restartWhenFinished') }}
+      </p>
+
+      <Button
+        class="place-self-end"
+        :label="t('menuLabels.Restart')"
+        severity="warn"
+        icon="pi pi-refresh"
+        @click="restartApp('Manual configuration complete')"
+      />
+    </div>
+  </BaseViewTemplate>
+</template>
+
+<script setup lang="ts">
+import Button from 'primevue/button'
+import Panel from 'primevue/panel'
+import Tag from 'primevue/tag'
+import { onMounted, ref } from 'vue'
+import { useI18n } from 'vue-i18n'
+
+import { electronAPI } from '@/utils/envUtil'
+import BaseViewTemplate from '@/views/templates/BaseViewTemplate.vue'
+
+const { t } = useI18n()
+
+const electron = electronAPI()
+
+const basePath = ref<string | null>(null)
+const sep = ref<'\\' | '/'>('/')
+
+const restartApp = (message?: string) => electron.restartApp(message)
+
+onMounted(async () => {
+  basePath.value = await electron.getBasePath()
+  if (basePath.value.indexOf('/') === -1) sep.value = '\\'
+})
+</script>
+
+<style scoped>
+.p-tag {
+  --p-tag-gap: 0.5rem;
+}
+
+.comfy-installer {
+  margin-top: max(1rem, max(0px, calc((100vh - 42rem) * 0.5)));
+}
+</style>