load template workflow via URL query param (#6546)

## Summary

Adds support for loading templates via URL query parameters. Users can
now share direct links to templates.

To test:

1. checkout this branch
2. start dev server on port 5173
3. go to http://localhost:5173/?template=image_qwen_image_edit_2509

**Examples:**
- `/?template=default` - Loads template with default source
- `/?template=flux_simple&source=custom` - Loads from custom source

Includes error handling with toast notifications for invalid templates
and comprehensive test coverage.

---------

Co-authored-by: Christian Byrne <c.byrne@comfy.org>

src/views/GraphView.vue

@@ -53,6 +53,7 @@ import { useSettingStore } from '@/platform/settings/settingStore'
 import { useTelemetry } from '@/platform/telemetry'
 import { useFrontendVersionMismatchWarning } from '@/platform/updates/common/useFrontendVersionMismatchWarning'
 import { useVersionCompatibilityStore } from '@/platform/updates/common/versionCompatibilityStore'
+import { useTemplateUrlLoader } from '@/platform/workflow/templates/composables/useTemplateUrlLoader'
 import type { StatusWsMessageStatus } from '@/schemas/apiSchema'
 import { api } from '@/scripts/api'
 import { app } from '@/scripts/app'
@@ -80,6 +81,9 @@ setupAutoQueueHandler()
 useProgressFavicon()
 useBrowserTabTitle()
 
+// Template URL loading
+const { loadTemplateFromUrl } = useTemplateUrlLoader()
+
 const { t } = useI18n()
 const toast = useToast()
 const settingStore = useSettingStore()
@@ -349,6 +353,9 @@ const onGraphReady = () => {
       tabCountChannel.postMessage({ type: 'heartbeat', tabId: currentTabId })
     }
 
+    // Load template from URL if present
+    void loadTemplateFromUrl()
+
     // Setting values now available after comfyApp.setup.
     // Load keybindings.
     wrapWithErrorHandling(useKeybindingService().registerUserKeybindings)()