Rework desktop install / startup UX (#5292)

### Summary

Complete redesign of Desktop app installer, implementing the new app
startup progress system and error reporting.

src/components/common/StartupDisplay.vue

@@ -0,0 +1,71 @@
+<template>
+  <div :class="wrapperClass">
+    <div class="grid grid-rows-2 gap-8">
+      <!-- Top container: Logo -->
+      <div class="flex items-end justify-center">
+        <img
+          src="/assets/images/comfy-brand-mark.svg"
+          :alt="t('g.logoAlt')"
+          class="w-60"
+        />
+      </div>
+      <!-- Bottom container: Progress and text -->
+      <div class="flex flex-col items-center justify-center gap-4">
+        <ProgressBar
+          v-if="!hideProgress"
+          :mode="progressMode"
+          :value="progressPercentage ?? 0"
+          :show-value="false"
+          class="w-90 h-2 mt-8"
+          :pt="{ value: { class: 'bg-brand-yellow' } }"
+        />
+        <h1 v-if="title" class="font-inter font-bold text-3xl text-neutral-300">
+          {{ title }}
+        </h1>
+        <p v-if="statusText" class="text-lg text-neutral-400">
+          {{ statusText }}
+        </p>
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import ProgressBar from 'primevue/progressbar'
+import { computed } from 'vue'
+import { useI18n } from 'vue-i18n'
+
+const { t } = useI18n()
+
+/** Props for the StartupDisplay component */
+interface StartupDisplayProps {
+  /** Progress: 0-100 for determinate, undefined for indeterminate */
+  progressPercentage?: number
+  /** Main title text */
+  title?: string
+  /** Status text shown below the title */
+  statusText?: string
+  /** Hide the progress bar */
+  hideProgress?: boolean
+  /** Use full screen wrapper (default: true) */
+  fullScreen?: boolean
+}
+
+const {
+  progressPercentage,
+  title,
+  statusText,
+  hideProgress = false,
+  fullScreen = true
+} = defineProps<StartupDisplayProps>()
+
+const progressMode = computed(() =>
+  progressPercentage === undefined ? 'indeterminate' : 'determinate'
+)
+
+const wrapperClass = computed(() =>
+  fullScreen
+    ? 'flex items-center justify-center min-h-screen'
+    : 'flex items-center justify-center'
+)
+</script>