Introduce tablet-optimized layouts using a NavigationRail on larger screens and constrained content widths for improved readability. Changes: - Add ResponsiveUtils utility class with tablet detection (600dp breakpoint) and maxContentWidth constant (640px for optimal line length) - Replace bottom navigation with NavigationRail on tablets in MainShellScreen - Constrain feed content to maxContentWidth on tablets across all screens: - FeedPage, CommunityFeedScreen, ProfileScreen posts/comments - PostDetailScreen post card and comment threads - CommunitiesScreen placeholder content - Use wrapForTablet helper for consistent responsive wrapping Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>