Add 'Explain This' page, surface backend-unavailable messages, and UI/UX fixes

From the PR description

Motivation

• Provide a new UI flow for explaining legal text in plain English so users can paste clauses and get an explanation.
• Make it clear to users when certain flows depend on a connected backend by surfacing a consistent backend-unavailable message.
• Fix small UI and state issues around the sidebar, upload error handling, and developer auth bypass visibility.

Description

• Add a new client page ExplainThisPage (/explain) which builds a structured prompt from user input and starts a chat via handleNewChat.
• Replace ambiguous error text in draft/page.tsx and review/page.tsx with a consistent "This will be available once GaryOSS is connected to the backend." message when chats cannot be started.
• Update InitialView to add an "Explain This" action, handle upload errors with an uploadError state and message, and expand the action grid to lg:grid-cols-5 to accommodate the new card.
• Fix sidebar persistence bug by writing sidebarOpenDesktop to localStorage in layout.tsx, rename the sidebar item label to Ask Gary in AppSidebar.tsx, and show a dev auth bypass notice on login and signup when NEXT_PUBLIC_GARY_SKIP_AUTH is true.

Testing

• No automated tests were run for this change.

Codex Task