feat: add live preview method setting for prompt execution (#7385)

## Summary

Add frontend setting to override live preview method per prompt
execution.

## Changes

- **What**: New setting `Comfy.Execution.PreviewMethod` allows users to
override preview method (default/none/auto/latent2rgb/taesd) from
frontend. Applied to Queue Prompt, Queue Front, Run Selected Nodes, and
Auto Queue.
- **Dependencies**: Requires backend support from
comfyanonymous/ComfyUI#11261

## Review Focus

- `'default'` option does not send `preview_method` to backend (uses
server CLI setting)
- Legacy UI intentionally not modified (deprecated, maintains backward
compatibility)
- `versionAdded: '1.35.3'` assigned tentatively; adjust as needed for
actual release version

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7385-feat-add-live-preview-method-setting-for-prompt-execution-2c66d73d365081759c9cebaec29f451c)
by [Unito](https://www.unito.io)

src/platform/settings/constants/coreSettings.ts

@@ -807,6 +807,17 @@ export const CORE_SETTINGS: SettingParams[] = [
     defaultValue: 64,
     versionAdded: '1.4.12'
   },
+  {
+    id: 'Comfy.Execution.PreviewMethod',
+    category: ['Comfy', 'Execution', 'PreviewMethod'],
+    name: 'Live preview method',
+    tooltip:
+      'Live preview method during image generation. "default" uses the server CLI setting.',
+    type: 'combo',
+    options: ['default', 'none', 'auto', 'latent2rgb', 'taesd'],
+    defaultValue: 'default',
+    versionAdded: '1.36.0'
+  },
   {
     id: 'LiteGraph.Canvas.MaximumFps',
     name: 'Maximum FPS',