Add handoff docs, UI primitives, Explain flow, font migration, and backend-unavailable messaging

From the PR description

Motivation

• Provide handoff and operational documentation to capture architecture, deployment status, and roadmap for the project.
• Surface additional assistant workflows (Explain, Review, Draft) and surface clearer UX for when the backend is not connected.
• Move away from runtime next/font/google usage toward local font assets and add small UI primitives to standardize component usage.

Description

• Added handoff and project docs: docs/HANDOFF.md, docs/ARCHITECTURE.md, and docs/DEPLOYMENT_STATUS.md, and a recommended docs/ROADMAP.md.
• Introduced an Explain flow and page at frontend/src/app/(pages)/explain/page.tsx and wired an Explain action into the assistant initial view.
• Updated Review and Draft pages to show a standardized backend-unavailable message when handleNewChat cannot start a chat, and added the same fallback for upload failure UI in InitialView.tsx.
• Added many UI primitive components under frontend/src/components/ui/ (card, alert, label, separator, tabs, textarea, skeleton) and imported them into pages.
• Migrated font strategy: removed next/font/google usage from frontend/src/app/layout.tsx, added @fontsource/inter in frontend/package.json, and imported Inter weights in frontend/src/app/globals.css while updating CSS font variables.
• Minor UI and behavior fixes: corrected sidebar localStorage handling in layout.tsx, changed assistant sidebar label to Ask Gary, adjusted the initial action grid sizing, and added dev-auth bypass notice on login/signup when NEXT_PUBLIC_GARY_SKIP_AUTH=true.

Testing

• Built the frontend app with npm run build --prefix frontend to validate bundling after the font and component changes (build succeeded).
• Performed basic smoke interactions in local dev (npm run dev --prefix frontend) to verify navigation to Explain, Review, and Draft shells and to confirm backend-unavailable messaging appears when handleNewChat returns no chat id (smoke checks passed).

Codex Task