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/platform/workflow/templates/composables/useTemplateUrlLoader.ts

@@ -0,0 +1,96 @@
+import { useToast } from 'primevue/usetoast'
+import { useI18n } from 'vue-i18n'
+import { useRoute } from 'vue-router'
+
+import { useTemplateWorkflows } from './useTemplateWorkflows'
+
+/**
+ * Composable for loading templates from URL query parameters
+ *
+ * Supports URLs like:
+ * - /?template=flux_simple (loads with default source)
+ * - /?template=flux_simple&source=custom (loads from custom source)
+ *
+ * Input validation:
+ * - Template and source parameters must match: ^[a-zA-Z0-9_-]+$
+ * - Invalid formats are rejected with console warnings
+ */
+export function useTemplateUrlLoader() {
+  const route = useRoute()
+  const { t } = useI18n()
+  const toast = useToast()
+  const templateWorkflows = useTemplateWorkflows()
+
+  /**
+   * Validates parameter format to prevent path traversal and injection attacks
+   */
+  const isValidParameter = (param: string): boolean => {
+    return /^[a-zA-Z0-9_-]+$/.test(param)
+  }
+
+  /**
+   * Loads template from URL query parameters if present
+   * Handles errors internally and shows appropriate user feedback
+   */
+  const loadTemplateFromUrl = async () => {
+    const templateParam = route.query.template
+
+    if (!templateParam || typeof templateParam !== 'string') {
+      return
+    }
+
+    // Validate template name format
+    if (!isValidParameter(templateParam)) {
+      console.warn(
+        `[useTemplateUrlLoader] Invalid template parameter format: ${templateParam}`
+      )
+      return
+    }
+
+    const sourceParam = (route.query.source as string | undefined) || 'default'
+
+    // Validate source parameter format
+    if (!isValidParameter(sourceParam)) {
+      console.warn(
+        `[useTemplateUrlLoader] Invalid source parameter format: ${sourceParam}`
+      )
+      return
+    }
+
+    // Load template with error handling
+    try {
+      await templateWorkflows.loadTemplates()
+
+      const success = await templateWorkflows.loadWorkflowTemplate(
+        templateParam,
+        sourceParam
+      )
+
+      if (!success) {
+        toast.add({
+          severity: 'error',
+          summary: t('g.error'),
+          detail: t('templateWorkflows.error.templateNotFound', {
+            templateName: templateParam
+          }),
+          life: 3000
+        })
+      }
+    } catch (error) {
+      console.error(
+        '[useTemplateUrlLoader] Failed to load template from URL:',
+        error
+      )
+      toast.add({
+        severity: 'error',
+        summary: t('g.error'),
+        detail: t('g.errorLoadingTemplate'),
+        life: 3000
+      })
+    }
+  }
+
+  return {
+    loadTemplateFromUrl
+  }
+}