feat: use cloud backend thumbnail resize for image previews (#9298)

## Summary

- In cloud mode, large generated images (4K, 8K+) cause browser freezing
when loaded at full resolution for preview display
- The cloud backend (ingest service) now supports a `res` query
parameter on `/api/view` that returns server-side resized JPEG (quality
80, max 512px) instead of redirecting to the full-size GCS original
- This PR adds `&res=512` to all image preview URLs in cloud mode,
reducing browser decode overhead from tens of MB to tens of KB
- Downloads still use the original resolution (no `res` param)
- No impact on localhost/desktop builds (`isCloud` compile-time
constant)

### without `?res`

302 -> png downloads
<img width="808" height="564" alt="스크린샷 2026-02-28 오후 6 53 03"
src="https://github.com/user-attachments/assets/7c1c62dd-0bc4-468d-9c74-7b98e892e126"
/>
<img width="323" height="137" alt="스크린샷 2026-02-28 오후 6 52 52"
src="https://github.com/user-attachments/assets/926aa0c4-856c-4057-96a0-d8fbd846762b"
/>

200 -> jpeg

### with `?res`
<img width="811" height="407" alt="스크린샷 2026-02-28 오후 6 51 55"
src="https://github.com/user-attachments/assets/d58d46ae-6749-4888-8bad-75344c4d868b"
/>


### Changes

- **New utility**: `getCloudResParam(filename?)` returns `&res=512` in
cloud mode for image files, empty string otherwise
- **Core stores**: `imagePreviewStore` appends `res` to node output
URLs; `queueStore.ResultItemImpl` gets a `previewUrl` getter (separates
preview from download URLs)
- **Applied to**: asset browser thumbnails, widget dropdown previews,
linear mode indicators, image compare node, background image upload

### Intentionally excluded

- Downloads (`getAssetUrl`) — need original resolution
- Mask editor — needs pixel-accurate data
- Audio/video/3D files — `res` only applies to raster images
- Execution-in-progress previews — use WebSocket blob URLs, not
`/api/view`

## Test plan

- [x] Unit tests for `getCloudResParam()` (5 tests: cloud/non-cloud,
image/non-image, undefined filename)
- [x] `pnpm typecheck` passes
- [x] `pnpm lint` passes
- [x] All 5332 unit tests pass
- [x] Manual verification on cloud.comfy.org: `res=512` returns 200 with
resized JPEG; without `res` returns 302 redirect to GCS PNG original

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/components/common/BackgroundImageUpload.vue

@@ -40,6 +40,7 @@ import InputText from 'primevue/inputtext'
 import { ref } from 'vue'
 
 import Button from '@/components/ui/button/Button.vue'
+import { appendCloudResParam } from '@/platform/distribution/cloudPreviewUtil'
 import { useToastStore } from '@/platform/updates/common/toastStore'
 import { api } from '@/scripts/api'
 
@@ -88,6 +89,7 @@ const handleFileUpload = async (event: Event) => {
           type: 'input',
           subfolder: 'backgrounds'
         })
+        appendCloudResParam(params, file.name)
         modelValue.value = `/api/view?${params.toString()}`
       }
     } catch (error) {