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/locales/en/main.json

@@ -780,6 +780,9 @@
       "vramLowToHigh": "VRAM Usage (Low to High)",
       "modelSizeLowToHigh": "Model Size (Low to High)",
       "default": "Default"
+    },
+    "error": {
+      "templateNotFound": "Template \"{templateName}\" not found"
     }
   },
   "graphCanvasMenu": {