feat(landing): add light/dark theme toggle; persist choice in localSt…

[hukshh]

What’s Added

Added a theme toggle button (sun/moon icon) to the top-right corner of the landing page header.

Implemented default theme = Dark.

Implemented persistent theme preference using localStorage under the key:
drawdb:theme

Added support for system theme detection via:
window.matchMedia('(prefers-color-scheme: dark)')

Added smooth but minimal transition animations with support for
prefers-reduced-motion.

Improved color variables for clean, consistent UI across both themes.

Made toggle accessible:

role="switch"

aria-checked

Keyboard focus (Tab, Enter, Space supported)

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