recipes: replace placeholder images with OKLAB dominant color backgrounds

Instead of generating/serving 20px placeholder images with blur CSS, extract
a perceptually accurate dominant color (Gaussian-weighted OKLAB average) and
use it as a solid background-color while the full image loads. Removes
placeholder image generation, blur CSS/JS, and placeholder directory references
across upload flows, API routes, service worker, and all card/hero components.
Adds admin bulk tool to backfill colors for existing recipes.

src/routes/api/[recipeLang=recipeLang]/edit/+server.ts

@@ -26,7 +26,7 @@ export const POST: RequestHandler = async ({request, locals}) => {
 
 	if (oldShortName !== newShortName) {
 		// Rename image files in all three directories
-		const imageDirectories = ['full', 'thumb', 'placeholder'];
+		const imageDirectories = ['full', 'thumb'];
 		const staticPath = join(process.cwd(), 'static', 'rezepte');
 
 		for (const dir of imageDirectories) {