Add a moon/sun button to the card header that switches the widget between
light and dark mode. The choice is stored in chrome.storage.local under
`theme`, synced live across Keka tabs, and applied before the first render
so there is no flash. Until the user picks one, it follows the OS
prefers-color-scheme.

- settings.js: loadTheme / saveTheme / onThemeChange
- main.js: load the saved theme at start and react to changes from other tabs
- ui.js: move hardcoded colours into CSS variables, add the dark palette,
  header theme button with aria-pressed/labels
- ui.js: derive --brand-soft from Keka's primary colour instead of its
  --primary-color-light, which can be dark and made the Plan "Today" strip
  and header badge unreadable in light mode
- README: document the toggle, permissions and file table