Add landing page with GitHub Pages workflow

[mode777]

What

• New static landing page under landing/ (plain HTML/CSS/JS, no build step): hero, feature grid, supported-formats table, self-host section with Docker snippet, footer credits + fan-project disclaimer.
• Brand mark + favicon as inline SVG using the app palette (navy #0b1326, cyan #4cd7f6, violet #7c3aed).
• New .github/workflows/docs.yml: builds a Pages artifact (landing at site root), runs a private-hostname leak scan (no-op unless PRIVATE_INSTANCE_HOSTNAME repo var is set) and a landing link checker; deploys on pushes to main / manual dispatch, PRs are check-only.

Why

Trackify has no public face. This replicates the PSflix landing + Pages pattern for a project home at https://mode777.github.io/trackify/ — independent of any running instance. Planned via OpenSpec change add-landing-page (user docs site follows in add-user-docs).

How to test

• CI: leak scan + link check run in this PR (check-only, no deploy).
• After merge: site live at the Pages URL; every local link resolves (verified locally, including a deliberately broken copy failing the check).
• Formats table cross-checked against web/player/backend_catalog.js.