fix: add null-safety for assets array in FileField component

Prevents crash when uploading images in Content panel 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/tinykit/studio/components/FileField.svelte

@@ -57,7 +57,7 @@
 
 			// Upload files
 			const updated = await pb.collection("_tk_projects").update(project_id, form)
-			const new_assets = (updated.assets as string[]).slice(before_count)
+			const new_assets = (updated.assets as string[] || []).slice(before_count)
 
 			if (multiple) {
 				const current = Array.isArray(value) ? value : value ? [value] : []