Theme selector: make the light/dark circles bigger

[gnacho]

Summary

The system/light/dark circles in the theme selector (header menu) are too small to see and hit comfortably. Make them bigger.

Context

The theme selector lives in src/ui/main_window.rs (ThemeSelector, added in #26 / v0.10.0). It follows the gnome-text-editor pattern: three GtkCheckButtons with CSS-drawn circles (radio) for follow, light and dark. The clickable button is 44x44 px but the visible circle is only 26x26 px.

Current behavior vs expected

• Current: checkbutton.theme-selector radio is 26x26 px (CSS at src/ui/main_window.rs:1156-1165) inside a 44x44 px button. The circle looks small, especially against the button's own background.
• Expected: a visibly larger circle, closer to the button area, so the three options are easier to distinguish and click.

Proposed approach

In the CSS at src/ui/main_window.rs:1148-1181, raise the min-width / min-height of checkbutton.theme-selector radio from 26 px to roughly 32-36 px. The hover ring and the checked overlay scale with it. Verify the follow (half-filled) gradient and the check icon still look right at the larger size.

Notes

• Cosmetic change; no logic changes expected in ThemeSelector::new.
• Keep the button hit area at 44 px (GNOME touch target) or grow it together with the circle.