Frontend walkthrough polish and bugfix pass

From the PR description

Motivation

• Improve the V1 authenticated walkthrough (Log in/bypass → Upload document → Ask Gary → Get cited answer) by fixing UX friction, broken routes, unclear labels, and opaque backend errors without adding new backend code.
• Make the dev auth bypass visible but unobtrusive so developers can test the flow safely while not permanently changing authentication behavior.
• Surface friendly, non-technical messaging for features that cannot function until the backend is connected.
• Keep changes small and frontend-only, avoiding API/DB/schema changes or new provider keys.

Description

• Surface upload failures in the assistant entry view by adding an uploadError state and friendly fallback copy so uploads fail with: This will be available once GaryOSS is connected to the backend. (src/app/components/assistant/InitialView.tsx).
• Rename the primary sidebar item from Assistant to Ask Gary to match the intended user journey and make the main action clearer (src/app/components/shared/AppSidebar.tsx).
• Fix desktop sidebar persistence by saving the correct desktop state variable (isSidebarOpenDesktop) to localStorage so the open/closed preference is preserved correctly (src/app/(pages)/layout.tsx).
• Replace generic "Could not start... Try again." errors in Review and Draft flows with the friendly backend-unavailable message so users see a clear explanation when backend-dependent actions cannot complete (src/app/(pages)/review/page.tsx, src/app/(pages)/draft/page.tsx).
• Add a small, nonintrusive dev-bypass notice on the auth pages when NEXT_PUBLIC_GARY_SKIP_AUTH=true so the bypass is obvious but doesn't dominate the UI (src/app/login/page.tsx, src/app/signup/page.tsx).
• No backend files, API contracts, or database schema were changed; all edits are frontend-only and scoped to UX polishing and safety messaging.

Testing

• Ran npm run lint in frontend which executed but surfaced many pre-existing lint errors across the repo; this PR did not introduce new lint rules and the lint run failed due to unrelated, repo-wide issues (output recorded).
• Ran npm run build in frontend which failed in this environment due to inability to fetch Google Fonts during the Next.js/Turbopack build, causing the production build to error (network/font fetch limitation in CI environment).
• No backend automated tests were run as this change intentionally avoids backend/deployment work and only adds user-facing fallback copy.

Codex Task