* feat: add theme switcher with system/light/dark segmented control
Add a three-way theme toggle (system, light, dark) as a segmented
icon control in the account dropdown menu. Rename placeholder daisyUI
theme names from "todo" to "openseo".
* refactor: always resolve data-theme explicitly, remove duplicated dark CSS
Instead of removing data-theme in "system" mode and relying on
@media (prefers-color-scheme: dark) CSS, always resolve to an
explicit theme name via matchMedia. This eliminates the duplicated
dark-mode CSS overrides.
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