fix: make Template.importUrl nullable+optional for user templates

User templates omit `import_url` entirely from the response (see
map_user_template_to_contentful_response in todoist/apps/import_export/
controllers/common.py), so the schema needs to accept undefined as well
as null. Surfaced by a live `getTemplates({ templateSource: 'user' })`
call, which was failing with "expected string, received undefined".

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: scottlovegrove

src/types/templates/types.ts

@@ -55,7 +55,7 @@ export const TemplateSchema = z.object({
     shortDescription: z.string(),
     longDescription: z.string(),
     instructions: z.string().nullable(),
-    importUrl: z.string().nullable(),
+    importUrl: z.string().nullable().optional(),
     viewType: z.enum(PROJECT_VIEW_STYLES).optional(),
     thumbnailImage: z.string().nullable(),
     thumbnailImageDark: z.string().nullable(),