feat: icon polish round, tour in Settings, big-icon Features, uncropped scenes

[Razee4315]

What (user feedback round)

App

• "Replay the welcome tour" button in Settings -> About with the waving mascot (was palette-only before); wired via a marklite:replay-tour event that creates a buffer first if none is open
• Icon set now used inside the app: keyboard icon in the shortcuts cheatsheet header, clock icon in document statistics, paper-plane + export icons in the Export menu, check-badge in success toasts

Website

• Features section rebuilt from the ruled matrix into a 3-column card grid with the illustrated icons at 64px, hover lift + icon wiggle (reduced-motion safe). The icon set finally shows its full potential
• Celebration scene no longer cropped: shown at its natural ratio in a contained max-w-3xl frame (its paper background blends with the page)
• OpenSource card gets the github-paper icon
• Contrast fixes for light theme: brand-soft text deepened in OpenSource code chip and footer NORMAL badge

Testing

tsc + vite build clean, 107/107 tests pass, Astro build clean. Screenshot-verified Features grid, Download section, and full page.