Add light theme support via prefers-color-scheme media query

The task web UI is dark-first with no light mode support. Users with system light theme preference see the dark theme regardless.

Add @media (prefers-color-scheme: light) CSS rules that override the dark color tokens with light equivalents.

Implementation:

• In Omni/Web/Style.hs, the color palette is defined as constants (cBg, cFg, cBgRaised, etc.)
• Add a prefers-color-scheme: light media query block that overrides CSS custom properties
• Approach: convert the hardcoded Haskell color constants to CSS custom properties (var(--bg), var(--fg), etc.), set dark values as default, override in the light media query
• If converting to CSS custom properties is too invasive, alternatively add a @media (prefers-color-scheme: light) block at the end of the CSS that targets the same selectors with light colors

Light palette suggestion (inverse of current ef-dream dark theme):

• Background: #faf8f6 (warm cream)
• Foreground: #232025 (current dark bg)
• Raised bg: #ffffff
• Border: #d4d0cc
• Muted: #6b6568
• Accent: keep current accent colors but slightly darker for contrast

Key files: Omni/Web/Style.hs, Omni/Task/Web/Style.hs Namespace: Omni/Task.hs