perf: disable Sentry, remove grain overlay and shader gradient files

- Disable Sentry in all 3 configs (client/server/edge) - replayIntegration
  was recording every DOM mutation causing overhead in Chrome
- Remove grain-overlay div and its CSS (SVG feTurbulence + mix-blend-mode:overlay
  forces software compositing in Chrome on every frame)
- Remove mix-blend-multiply from BackgroundBlobs (prevents Chrome GPU compositing)
- Delete unused Grain.tsx, ShaderGradientBackground.tsx and its client wrapper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sentry.edge.config.ts

@@ -5,12 +5,6 @@
 import * as Sentry from "@sentry/nextjs";
 
 Sentry.init({
-  // DSN from environment variable with fallback to wizard-provided value
   dsn: process.env.NEXT_PUBLIC_SENTRY_DSN || "https://148e31ea874c60f3d2e0f70fd6701f6b@o4510751135105024.ingest.de.sentry.io/4510751388926032",
-
-  // Define how likely traces are sampled. Adjust this value in production, or use tracesSampler for greater control.
-  tracesSampleRate: process.env.NODE_ENV === "production" ? 0.1 : 1,
-
-  // Setting this option to true will print useful information to the console while you're setting up Sentry.
-  debug: false,
+  enabled: false,
 });