Polish Codesemble landing page UX and visuals

[pate0304]

What changed

• replace the dense long-scroll narrative with a clearer hero, four-step workflow, illustrative team explorer, safety/output section, install CTA, and focused docs
• fix the stylesheet artifact that left literal + patch markers in the live CSS
• add progressive-enhancement-safe interactions: mobile menu escape/outside-click handling, accessible coverage tabs, signal buttons, and clipboard status
• add a calmer ImageGen repository-to-team visual and social metadata
• regenerate source checksums for the new project-bound asset

Validation

• node --check site/script.js
• npm run checksums:verify
• git diff --check
• local HTTP smoke checks for page, CSS, JS, and generated visual