feat: mii edit UX, a11y fixes, and frontend unit tests

- API: return success message on mii edit
- FE: flash banner + helpers, submit response helper, Vitest
- A11y: labels, alt text, banner close; css checkerboard class
- DX: root tsconfig refs, strict/tsconfig, next dev --webpack

Made-with: Cursor

backend/package.json

@@ -4,7 +4,7 @@
 	"private": true,
 	"packageManager": "pnpm@10.33.0",
 	"scripts": {
-		"dev": "next dev",
+		"dev": "next dev --webpack",
 		"build": "next build",
 		"start": "next start",
 		"lint": "next lint",

backend/src/app/api/mii/[id]/edit/route.ts

@@ -281,5 +281,8 @@ export async function POST(request: NextRequest, { params }: { params: Promise<{
 		console.error("Cloudflare cache purge failed:", err);
 	});
 
-	return rateLimit.sendResponse({ success: true });
+	return rateLimit.sendResponse({
+		success: true,
+		message: "Mii updated successfully.",
+	});
 }