fix: add null-safety for assets array in file upload

Prevents 500 error when creating/updating records with images
on projects where updated.assets may be null/undefined.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: elemdos

src/routes/_tk/data/[project_id]/[collection]/+server.ts

@@ -263,7 +263,7 @@ async function upload_files_to_assets(
 
 	// Upload all files at once
 	const updated = await pb.collection('_tk_projects').update(project_id, upload_form)
-	const new_assets = (updated.assets as string[]).slice(before_count)
+	const new_assets = (updated.assets as string[] || []).slice(before_count)
 
 	// Map uploaded filenames back to fields
 	let new_index = 0

src/routes/_tk/data/[project_id]/[collection]/[id]/+server.ts

@@ -226,7 +226,7 @@ async function upload_files_to_assets(
 	const before_count = (project.assets as string[] || []).length
 
 	const updated = await pb.collection('_tk_projects').update(project_id, upload_form)
-	const new_assets = (updated.assets as string[]).slice(before_count)
+	const new_assets = (updated.assets as string[] || []).slice(before_count)
 
 	let new_index = 0
 	for (const [field_name, indices] of field_indices) {