Add SECURITY.md, update CONTRIBUTING, issue templates, Lighthouse fixes

frontend/index.html

@@ -8,6 +8,7 @@
          tighten by moving it to a separate .js file if desired. -->
     <meta http-equiv="Content-Security-Policy" content="default-src 'self'; script-src 'self' 'unsafe-inline' 'unsafe-eval' 'wasm-unsafe-eval'; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com; img-src 'self' data: blob:; media-src 'self' blob:; connect-src 'self' https://gleitz.github.io https://danigb.github.io https://goldst.dev; worker-src 'self' blob:; object-src 'none'; base-uri 'self'" />
     <title>Bach to Basics</title>
+    <meta name="description" content="Browser-based piano practice tool. Import MIDI, MusicXML, or PDF sheet music and learn pieces with falling notes, interactive piano, A/B loop, speed trainer, and wait mode." />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <link rel="icon" type="image/png" href="/favicon-light.png" />
     <link rel="apple-touch-icon" href="/apple-touch-icon.png" />