style: refresh light theme surfaces

[Willxup]

Summary

• replace the warm light-theme grays with a cool slate and blue-white surface palette
• clarify header and sidebar chrome while reducing background artwork bleed-through
• refine the upload dropzone, empty queue, and queued-file cards without changing the dark theme

Verification

• make check
• npm --prefix web run e2e (2 passed)
• logged-in desktop visual review in light and dark themes; no console errors