Create a global onboarding context that any write action (likes, replies, new topics) can use to check onboarding completion before proceeding. Previously only the New Topic page had this gate, causing 403 errors when unboarded users tried to like or reply. - Add OnboardingProvider context with ensureOnboarded() fail-open gate - Add shared test utility (mock-onboarding.tsx) - Gate LikeButton.handleToggle with ensureOnboarded() - Gate ReplyComposer.handleSubmit with ensureOnboarded() - Migrate New Topic page from standalone hook to context - Wire OnboardingProvider into layout.tsx provider stack - Add onboarding context mocks to transitive test files