Make /wordart responsive and fix atlas background warning

[apresmoi]

Summary

• make the /wordart composer responsive like /gallery: below 760px the two floating panels collapse into bottom sheets toggled one-at-a-time by a Style / Controls tab bar over a full-bleed stage (desktop keeps both panels floating, unchanged)
• scope the mobile collapse to .wa-gui:not(.wa-gui--inline) so the left card's own inline lil-gui keeps showing inside the card
• stop the React atlas leaf from listing the background shorthand alongside the background-* longhands in one inline style object — React warned on every re-render (mixing shorthand/longhand); mirrors the existing Vue renderer fix, vanilla builds a CSS string so is unaffected

Validation

• pnpm test && pnpm build
• Playwright at /wordart (390px): tabs toggle each sheet, Style shows text/font/type/color/presets, Controls shows shape/layout/camera/lighting, both clear the tab bar, Esc closes; desktop + 800px keep both panels floating with no tab bar
• Playwright at /wordart (1366px): shorthand/conflict console warnings drop from hundreds to 0, no other errors