Select Color: frameless gradient button, hex beside H/S/B, all Options swatches

- The gradient mode button is just its gradient, without a frame.
- In the gradient view H, S and B sit on the left and Hex on the right
  of one row; the nearest index colour moves to the row below.
- Every colour swatch in Options opens the Select Color panel: the six
  theme colours, crosshair, snap marker and command line text join the
  three viewport backgrounds. BgTarget becomes UiColor, and one
  color_swatch helper replaces the four hand-built swatches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

src/app/update/mod.rs

@@ -11061,7 +11061,7 @@ impl OpenCADStudio {
                 // the recent list.
                 if !matches!(
                     self.color_picker.as_ref().map(|picker| &picker.target),
-                    Some(crate::app::ColorPickTarget::Background(_))
+                    Some(crate::app::ColorPickTarget::Interface(_))
                 ) {
                     self.note_recent_color(color);
                 }