feat: adopt Signal website identity

[TheRealBecks]

Replace the oversized homepage and cargo-route identity with the Signal design and Exchange mark. Supported format chips and guide cards now render from one registry, allowing the homepage to grow to five or six formats without publishing an exhaustive route matrix.

The production registry continues to list only Docker Compose, Podman resources, and Quadlet. Docker resources, Kubernetes, and the optional Helm case are covered by private layout fixtures until their public documentation exists.

Changes

• add the dedicated Zensical homepage template and theme-aware Exchange logo
• apply the charcoal/lime Signal palette with an accessible light theme
• render format chips and input-guide cards from project.extra.formats
• verify format inventory, guide destinations, unique IDs, public routes, and the keyboard skip target
• record the scalable discovery decision and update brand/customization documentation

Validation

• ./scripts/check-all.sh — all 18 steps passed
• 105 unit tests passed
• strict Zensical build, Rust API documentation, static route/privacy checks, generated/local link checks, and whitespace checks passed
• browser checks cover 320, 390, 768, 1440, and 1920 px widths, plus five- and six-format fixtures

Closes #75