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/MigrationPicker.vue

@@ -0,0 +1,130 @@
+<template>
+  <div class="flex flex-col gap-6 w-[600px]">
+    <!-- Source Location Section -->
+    <div class="flex flex-col gap-4">
+      <p class="text-neutral-400 my-0">
+        {{ $t('install.migrationSourcePathDescription') }}
+      </p>
+
+      <div class="flex gap-2">
+        <InputText
+          v-model="sourcePath"
+          :placeholder="$t('install.locationPicker.migrationPathPlaceholder')"
+          class="flex-1"
+          :class="{ 'p-invalid': pathError }"
+          @update:model-value="validateSource"
+        />
+        <Button icon="pi pi-folder" class="w-12" @click="browsePath" />
+      </div>
+
+      <Message v-if="pathError" severity="error">
+        {{ pathError }}
+      </Message>
+    </div>
+
+    <!-- Migration Options -->
+    <div v-if="isValidSource" class="flex flex-col gap-4 p-4 rounded-lg">
+      <h3 class="text-lg mt-0 font-medium text-neutral-100">
+        {{ $t('install.selectItemsToMigrate') }}
+      </h3>
+
+      <div class="flex flex-col gap-3">
+        <div
+          v-for="item in migrationItems"
+          :key="item.id"
+          class="flex items-center gap-3 p-2 hover:bg-neutral-700 rounded"
+          @click="item.selected = !item.selected"
+        >
+          <Checkbox
+            v-model="item.selected"
+            :input-id="item.id"
+            :binary="true"
+            @click.stop
+          />
+          <div>
+            <label :for="item.id" class="text-neutral-200 font-medium">
+              {{ item.label }}
+            </label>
+            <p class="text-sm text-neutral-400 my-1">
+              {{ item.description }}
+            </p>
+          </div>
+        </div>
+      </div>
+    </div>
+
+    <!-- Skip Migration -->
+    <div v-else class="text-neutral-400 italic">
+      {{ $t('install.migrationOptional') }}
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { MigrationItems } from '@comfyorg/comfyui-electron-types'
+import Button from 'primevue/button'
+import Checkbox from 'primevue/checkbox'
+import InputText from 'primevue/inputtext'
+import Message from 'primevue/message'
+import { computed, ref, watchEffect } from 'vue'
+import { useI18n } from 'vue-i18n'
+
+import { electronAPI } from '@/utils/envUtil'
+
+const { t } = useI18n()
+
+const electron = electronAPI()
+
+const sourcePath = defineModel<string>('sourcePath', { required: false })
+const migrationItemIds = defineModel<string[]>('migrationItemIds', {
+  required: false
+})
+
+const migrationItems = ref(
+  MigrationItems.map((item) => ({
+    ...item,
+    selected: true
+  }))
+)
+
+const pathError = ref('')
+const isValidSource = computed(
+  () => sourcePath.value !== '' && pathError.value === ''
+)
+
+const validateSource = async (sourcePath: string | undefined) => {
+  if (!sourcePath) {
+    pathError.value = ''
+    return
+  }
+
+  try {
+    pathError.value = ''
+    const validation = await electron.validateComfyUISource(sourcePath)
+
+    if (!validation.isValid) pathError.value = validation.error ?? 'ERROR'
+  } catch (error) {
+    console.error(error)
+    pathError.value = t('install.pathValidationFailed')
+  }
+}
+
+const browsePath = async () => {
+  try {
+    const result = await electron.showDirectoryPicker()
+    if (result) {
+      sourcePath.value = result
+      await validateSource(result)
+    }
+  } catch (error) {
+    console.error(error)
+    pathError.value = t('install.failedToSelectDirectory')
+  }
+}
+
+watchEffect(() => {
+  migrationItemIds.value = migrationItems.value
+    .filter((item) => item.selected)
+    .map((item) => item.id)
+})
+</script>