Select Color: full-width Current/New band, Tab focus, tighter fields

- Current and New are a 50/50 band across the panel with their names
  and values inside; the Color text box is gone.
- The gradient mode button is one gradient across the whole button,
  gray until that mode is active. Mode buttons share one width.
- Tab and Shift+Tab move between the panel's own fields.
- Number boxes are 54 px wide, the hex box 74 px.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

src/app/update/mod.rs

@@ -397,6 +397,11 @@ impl OpenCADStudio {
                     Some("ENTER" | "RETURN") => {
                         return self.update(Message::ColorPicker(ColorPickerMsg::Ok));
                     }
+                    Some("TAB") => {
+                        return crate::ui::color_select::picker::focus_step(
+                            !key.contains("SHIFT+"),
+                        );
+                    }
                     _ => {}
                 },
                 Message::ArrowKeyPressed { direction, .. } => {