Add Chrome extension + convert Next.js to landing page #4035

Originally written by @ccantynz-alt on GitHub.
Imported from https://github.com/ccantynz-alt/48co.v1/pull/2

Chrome extension (extension/):

• Manifest V3 with content scripts for Claude, ChatGPT, Gemini, DeepSeek
• Site adapter pattern with tiered selector fallbacks per AI chat platform
• Shadow DOM widget injection (isolated from host page CSS, z-index max)
• document.execCommand('insertText') for ProseMirror text injection
• MutationObserver for SPA element detection (not polling)
• Offscreen document for mic access (MV3 requirement)
• Hybrid speech: Web Speech API (free) + Whisper API (premium BYOK)
• 8 voice commands: refactor/explain/debug/fix/test/optimize/send/cancel
• Auto coding mode with keyword detection + language inference
• Scroll wheel toggle, click toggle, Ctrl+Shift+Space shortcut
• Draggable floating widget with 4 states (idle/recording/processing/done)
• Settings popup with engine selector, toggle switches, type speed slider
• PNG icons generated from SVG sources

Landing page (app/):

• Convert HUD demo to marketing landing page for 48co.nz
• Hero section with interactive waveform demo
• Feature grid, voice commands reference, competitor comparison table
• Install instructions for manual Chrome extension loading
• SEO metadata updated

https://claude.ai/code/session_01KRs33M5QaFJVw6ZqLYszUV