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>

instrumentation-client.ts

@@ -5,28 +5,8 @@
 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",
-
-  // Add optional integrations for additional features
-  integrations: [Sentry.replayIntegration()],
-
-  // 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,
-  // Enable logs to be sent to Sentry
-  enableLogs: true,
-
-  // Define how likely Replay events are sampled.
-  // This sets the sample rate to be 10%. You may want this to be 100% while
-  // in development and sample at a lower rate in production
-  replaysSessionSampleRate: 0.1,
-
-  // Define how likely Replay events are sampled when an error occurs.
-  replaysOnErrorSampleRate: 1.0,
-
-  // Enable sending user PII (Personally Identifiable Information)
-  // https://docs.sentry.io/platforms/javascript/guides/nextjs/configuration/options/#sendDefaultPii
-  sendDefaultPii: true,
+  enabled: false,
 });
 
 export const onRouterTransitionStart = Sentry.captureRouterTransitionStart;