ui: single rounding knob, fix balloon-shaped cards

New settings/ui.nix holds `radius` as the one source of truth. Theme
derives radiusCard/Small/Tiny from it and hyprland's decoration.rounding
reads the same value, so one number rescales the whole desktop —
previously the shell and the window corners were unrelated constants,
which is why doubling Theme.radius left windows at 8.

Cards were using the SAME radius as the panel containing them. Nested
corners need outer − gap; matching them makes the inner corner
over-round for its size, which is the inflated/balloon look. All nine
inner surfaces now use radiusCard (radius − panelGap). Invisible at
r=8, obvious at r=32.

Radius 32 -> 24.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

settings/hyprland.nix

@@ -3,6 +3,7 @@
 let
   isMacbook = config.networking.hostName == "FredOS-Macbook";
   isGaming  = !isMacbook;
+  ui = import ./ui.nix;   # shared corner-rounding knob
 
 in
 {
@@ -114,7 +115,8 @@ in
             };
 
             decoration = {
-              rounding = 8;
+              # tracks Theme.radius via settings/ui.nix
+              rounding = ui.radius;
               blur = {
                 enabled = true;
                 size = 1;