perf(assets): convert images to webp and check asset paths

Convert 26 PNG/JPG assets to WebP: max 1600 px, q80, diagrams lossless.
src/assets shrinks from 8.3 MB to 4.2 MB.
Use me.jpg for og:image and twitter:image; some crawlers skip WebP.
Add scripts/check-assets.mjs to lint, so a wrong asset path fails lint,
not only at runtime.

scripts/check-assets.mjs

@@ -0,0 +1,37 @@
+#!/usr/bin/env node
+// Verifies that every asset path in AssetsConstants.ts and index.html exists.
+// A wrong path builds fine and only 404s at runtime.
+
+import { existsSync, readFileSync, readdirSync } from 'node:fs';
+import { join, dirname, extname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const srcDir = join(dirname(fileURLToPath(import.meta.url)), '..', 'src');
+const EXTENSIONS = new Set(['.ts', '.html', '.scss', '.json']);
+
+function* walk(dir) {
+  for (const entry of readdirSync(dir, { withFileTypes: true })) {
+    const path = join(dir, entry.name);
+    if (entry.isDirectory()) {
+      yield* walk(path);
+    } else if (EXTENSIONS.has(extname(path)) && !path.endsWith('.spec.ts')) {
+      yield path;
+    }
+  }
+}
+
+const missing = [];
+for (const file of walk(srcDir)) {
+  const content = readFileSync(file, 'utf8');
+  for (const [, path] of content.matchAll(/\b(assets\/[\w./-]+)/g)) {
+    if (!existsSync(join(srcDir, path))) {
+      missing.push(`${file}: ${path}`);
+    }
+  }
+}
+
+if (missing.length > 0) {
+  console.error(`Missing assets:\n${missing.join('\n')}`);
+  process.exit(1);
+}
+console.log('All referenced assets exist.');