refactor: harden site consent, freshness, and deploy integrity

[Dingding-leo]

Direction

Preserve the existing Last Blue Hour visual identity. This is a trust, performance, maintainability, and deployment-integrity pass rather than a redesign.

Visitor experience

• make ambience opt-in for first-time visitors; an intentional Play action is now required before sound starts
• retain the saved preference for visitors who previously enabled ambience
• clarify Austin's public identity as a dental student in Adelaide and independent builder without changing the cinematic presentation
• keep native sharing hydration-safe and remove an invalid decorative ARIA attribute

Fresh deployments and offline behavior

• version the service-worker registration with the current build SHA
• reload once when a new worker replaces an already-controlled page, preventing stale HTML from requesting deleted chunks
• isolate page/static caches per build while keeping a bounded reusable media cache
• leave streamed audio/video and byte-range requests to the browser
• preserve the independent /KnightClub/ and /Denki/ service-worker scopes

Code and dependency health

• upgrade and pin Next.js to 16.3.1
• resolve the production dependency advisories found by npm audit
• restore supported ESLint 9 + eslint-config-next validation
• remove the unused 1,459-line legacy Site.tsx
• remove the obsolete synthetic sound engine; the live experience continues to use the recorded ambience engine
• simplify duplicated hydration and mount-only state paths in sharing, projects, audio volume, and chapter selection

Static-export integrity

• add an export validator that checks internal links, assets, srcset entries, hash fragments, required SEO/PWA files, manifest JSON, and service-worker syntax
• make npm run check run lint, TypeScript, the complete static export, and export validation
• require a clean high-severity production audit before CI or GitHub Pages deployment succeeds
• standardize CI and deployment on Node.js 22 with a build-specific service-worker ID

Verification

The branch has already passed source linting, TypeScript, the Next.js 16 static export, validation of 20 pages and 1,213 internal references, service-worker syntax checks, and a clean production dependency audit.

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