The sidebar's bottom block gets two new controls:
- A "What's New" button that opens the release-note popup, with a badge
until the operator has read the latest release.
- A Dark Mode section that replaces the old one-click toggle with an
explicit Light/Dark switch, so the current mode is always visible
instead of being inferred from the button's label.
The theme colour picker stays in Settings -> Appearance; the sidebar
carries the mode switch only, so the nav rail is not a strip of 16
coloured squares.
