Add 'Explain This' page, switch to Inter font, and surface dev/back-end availability messaging

From the PR description

Motivation

• Introduce a lightweight "Explain This" workflow for breaking down legal language and surface UI messaging when the backend or dev bypass is not available.
• Replace the custom Google font handling with Inter to simplify font loading and ensure consistent sans-serif/serif fallbacks.
• Improve user feedback for document uploads and feature availability, and make the sidebar wording clearer for end users.

Description

• Added a new page at src/app/(pages)/explain/page.tsx that collects legal text, context, goal, and tone, builds a structured prompt, and starts a chat via handleNewChat or shows a backend-unavailable message.
• Updated the assistant initial view (InitialView.tsx) to include an "Explain This" action card, handle upload errors with uploadError state, and grow the action grid to accommodate the new card.
• Changed error copy in draft/page.tsx and review/page.tsx to a single BACKEND_UNAVAILABLE_MESSAGE informing users that the feature requires a connected backend.
• Switched font strategy: added @fontsource/inter to frontend/package.json, imported Inter in globals.css, removed next/font/google usage from layout.tsx, and updated CSS variables to use Inter as the default for --font-sans and --font-serif.
• Small UX/copy changes including renaming the sidebar item label from "Assistant" to "Ask Gary" in AppSidebar.tsx and displaying a dev auth bypass banner in login/page.tsx and signup/page.tsx when NEXT_PUBLIC_GARY_SKIP_AUTH is set.

Testing

• Ran a local frontend build with cd frontend && npm ci && npm run build which completed successfully.
• Ran linting with cd frontend && npm run lint and no lint errors were reported.

Codex Task