Walkthrough polish pass fixes localStorage bug and softens unreachable-backend errors

The bug fix is in (pages)/layout.tsx. The effect persisting the desktop sidebar open/closed state was writing isSidebarOpen.toString() - the mobile state variable - to localStorage, even though the effect dependency array already tracked isSidebarOpenDesktop. The corrected line reads localStorage.setItem("sidebarOpen", isSidebarOpenDesktop.toString()). One line, but a real correctness issue.

The error message changes across /draft, /review, and InitialView replace "Could not start the {draft|review}. Try again." with "This will be available once GaryOSS is connected to the backend." This is honest: on a demo deploy, handleNewChat returns null because there's no backend to call. The old copy implied a transient failure; the new copy names the actual cause. InitialView also gains an explicit uploadError state so upload failures surface inline rather than failing silently.

The sidebar label change ("Assistant" -> "Ask Gary") is product-specific. The login/signup dev-bypass notice complements the in-app banner from the auth bypass feature: when NEXT_PUBLIC_GARY_SKIP_AUTH=true, an amber block appears on those pages so developers can see auth is being skipped before they even log in.