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/InstallFooter.vue

@@ -0,0 +1,79 @@
+<template>
+  <div class="grid grid-cols-[1fr_auto_1fr] items-center gap-4">
+    <!-- Back button -->
+    <Button
+      v-if="currentStep !== '1'"
+      :label="$t('g.back')"
+      severity="secondary"
+      icon="pi pi-arrow-left"
+      class="font-inter rounded-lg border-0 px-6 py-2 justify-self-start"
+      @click="$emit('previous')"
+    />
+    <div v-else></div>
+
+    <!-- Step indicators in center -->
+    <StepList class="flex justify-center items-center gap-3 select-none">
+      <Step value="1" :pt="stepPassthrough">
+        {{ $t('install.gpu') }}
+      </Step>
+      <Step value="2" :disabled="disableLocationStep" :pt="stepPassthrough">
+        {{ $t('install.installLocation') }}
+      </Step>
+      <Step value="3" :disabled="disableSettingsStep" :pt="stepPassthrough">
+        {{ $t('install.desktopSettings') }}
+      </Step>
+    </StepList>
+
+    <!-- Next/Install button -->
+    <Button
+      :label="currentStep !== '3' ? $t('g.next') : $t('g.install')"
+      class="px-8 py-2 bg-brand-yellow hover:bg-brand-yellow/90 font-inter rounded-lg border-0 transition-colors justify-self-end"
+      :pt="{
+        label: { class: 'text-neutral-900 font-inter font-black' }
+      }"
+      :disabled="!canProceed"
+      @click="currentStep !== '3' ? $emit('next') : $emit('install')"
+    />
+  </div>
+</template>
+
+<script setup lang="ts">
+import type { PassThrough } from '@primevue/core'
+import Button from 'primevue/button'
+import Step, { type StepPassThroughOptions } from 'primevue/step'
+import StepList from 'primevue/steplist'
+
+defineProps<{
+  /** Current step index as string ('1', '2', '3', '4') */
+  currentStep: string
+  /** Whether the user can proceed to the next step */
+  canProceed: boolean
+  /** Whether the location step should be disabled */
+  disableLocationStep: boolean
+  /** Whether the migration step should be disabled */
+  disableMigrationStep: boolean
+  /** Whether the settings step should be disabled */
+  disableSettingsStep: boolean
+}>()
+
+defineEmits<{
+  previous: []
+  next: []
+  install: []
+}>()
+
+const stepPassthrough: PassThrough<StepPassThroughOptions> = {
+  root: { class: 'flex-none p-0 m-0' },
+  header: ({ context }) => ({
+    class: [
+      'h-2.5 p-0 m-0 border-0 rounded-full transition-all duration-300',
+      context.active
+        ? 'bg-brand-yellow w-8 rounded-sm'
+        : 'bg-neutral-700 w-2.5',
+      context.disabled ? 'opacity-60 cursor-not-allowed' : ''
+    ].join(' ')
+  }),
+  number: { class: 'hidden' },
+  title: { class: 'hidden' }
+}
+</script>