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/templates/BaseViewTemplate.vue

@@ -0,0 +1,52 @@
+<template>
+  <div
+    class="font-sans w-screen h-screen flex flex-col"
+    :class="[
+      dark
+        ? 'text-neutral-300 bg-neutral-900 dark-theme'
+        : 'text-neutral-900 bg-neutral-300'
+    ]"
+  >
+    <!-- Virtual top menu for native window (drag handle) -->
+    <div
+      v-show="isNativeWindow()"
+      ref="topMenuRef"
+      class="app-drag w-full h-(--comfy-topbar-height)"
+    />
+    <div class="grow w-full flex items-center justify-center overflow-auto">
+      <slot />
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { nextTick, onMounted, ref } from 'vue'
+
+import { electronAPI, isElectron, isNativeWindow } from '../../utils/envUtil'
+
+const { dark = false } = defineProps<{
+  dark?: boolean
+}>()
+
+const darkTheme = {
+  color: 'rgba(0, 0, 0, 0)',
+  symbolColor: '#d4d4d4'
+}
+
+const lightTheme = {
+  color: 'rgba(0, 0, 0, 0)',
+  symbolColor: '#171717'
+}
+
+const topMenuRef = ref<HTMLDivElement | null>(null)
+onMounted(async () => {
+  if (isElectron()) {
+    await nextTick()
+
+    electronAPI().changeTheme({
+      ...(dark ? darkTheme : lightTheme),
+      height: topMenuRef.value?.getBoundingClientRect().height ?? 0
+    })
+  }
+})
+</script>