feat: add welcome message for Linear Mode (#8562)

## Summary

Adds a welcome/onboarding message for App Mode (Linear Mode) that
displays when no workflow output is selected, targeting first-time
users.


| Before | After |
|
---------------------------------------------------------------------------------------------------------------------------------------------
|
---------------------------------------------------------------------------------------------------------------------------------------------
|
| <img width="1914" height="1084" alt="Screenshot from 2026-02-02
20-47-30"
src="https://github.com/user-attachments/assets/f3693422-b6ee-496a-a354-d9cede501330"
/> | <img width="1919" height="1085" alt="Screenshot from 2026-02-02
21-32-20"
src="https://github.com/user-attachments/assets/dbfda4d0-251c-4099-ad36-3d7c0220d264"
/> |


## Changes

- **LinearWelcome.vue**: New component with 4 sections explaining App
Mode interface, sharing workflows, and widget control
- **LinearPreview.vue**: Replace dimmed logo fallback with LinearWelcome
component
- **i18n**: Add `linearMode.welcome.*` translation keys

## Review Focus

- Copy targets users who have never used ComfyUI—assumes no knowledge of
nodes/graphs
- Component uses semantic Tailwind classes from design system

Fixes COM-14274

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8562-feat-add-welcome-message-for-Linear-Mode-2fc6d73d365081b2b736f211152e1cce)
by [Unito](https://www.unito.io)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added a comprehensive welcome screen in App Mode that introduces users
to the simplified interface hiding node graphs, explains the layout
structure with outputs and controls, provides instructions for sharing
workflows as simple tools, and guides users on customizing which
settings are visible through widget promotion.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/renderer/extensions/linearMode/LinearPreview.vue

@@ -11,6 +11,7 @@ import type { AssetItem } from '@/platform/assets/schemas/assetSchema'
 import { useWorkflowStore } from '@/platform/workflow/management/stores/workflowStore'
 import { extractWorkflowFromAsset } from '@/platform/workflow/utils/workflowExtractionUtil'
 import ImagePreview from '@/renderer/extensions/linearMode/ImagePreview.vue'
+import LinearWelcome from '@/renderer/extensions/linearMode/LinearWelcome.vue'
 // Lazy-loaded to avoid pulling THREE.js into the main bundle
 const Preview3d = () => import('@/renderer/extensions/linearMode/Preview3d.vue')
 import VideoPreview from '@/renderer/extensions/linearMode/VideoPreview.vue'
@@ -177,9 +178,5 @@ async function rerun(e: Event) {
     v-else-if="getMediaType(selectedOutput) === '3d'"
     :model-url="selectedOutput!.url"
   />
-  <img
-    v-else
-    class="pointer-events-none flex-1 max-h-full md:contain-size brightness-50 opacity-10"
-    src="/assets/images/comfy-logo-mono.svg"
-  />
+  <LinearWelcome v-else />
 </template>

src/renderer/extensions/linearMode/LinearWelcome.vue

@@ -0,0 +1,29 @@
+<script setup lang="ts">
+import { t } from '@/i18n'
+</script>
+
+<template>
+  <div
+    role="article"
+    data-testid="linear-welcome"
+    class="flex flex-col items-center justify-center h-full gap-6 p-8 max-w-lg mx-auto text-center"
+  >
+    <div class="flex flex-col gap-2">
+      <h2 class="text-xl font-semibold text-base-foreground">
+        {{ t('linearMode.welcome.title')
+        }}<sup class="ml-1 text-xs font-normal text-muted-foreground">{{
+          t('g.experimental')
+        }}</sup>
+      </h2>
+      <p class="text-base text-muted-foreground">
+        {{ t('linearMode.welcome.intro') }}
+      </p>
+    </div>
+
+    <div class="flex flex-col gap-3 text-xs text-muted-foreground/70 max-w-md">
+      <p>{{ t('linearMode.welcome.layout') }}</p>
+      <p>{{ t('linearMode.welcome.sharing') }}</p>
+      <p>{{ t('linearMode.welcome.widget') }}</p>
+    </div>
+  </div>
+</template>