Handoff docs, Explain flow, local font, and UI primitive expansion

The documentation commit (3a52bcd) adds docs/HANDOFF.md, docs/ARCHITECTURE.md, docs/DEPLOYMENT_STATUS.md, and docs/ROADMAP.md. These describe the current deployment topology (Workers for frontend, Railway/Render for backend), the secrets policy, and known blockers as of the handoff. The architecture doc explicitly calls out that the backend cannot go onto Workers due to Node-only dependencies (filesystem, child processes for LibreOffice, large in-memory buffers).

The font change swaps next/font/google - which fetches at runtime and fails in CI environments without external network access - for @fontsource/inter, a self-hosted copy bundled at install time. The DEPLOYMENT_STATUS.md lists this as a current blocker: npm install may fail in restricted environments if @fontsource/inter can't be fetched from npm. CSS font variables were updated to match.

The new UI primitives live in frontend/src/components/ui and follow the shadcn pattern. foolish-bandit validated the changes with a production build and smoke tests of the new shells, then self-merged in under a minute.

The Explain page addition is a new guided workflow shell alongside Review and Draft. It takes pasted legal text and returns an explanation of what it says, what could go wrong, and how it might be revised. The backend-unavailable fallback messaging is consistent with the rest of the polish passes.