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/mod.rs

@@ -1577,12 +1577,17 @@ pub struct SaveOutcome {
     refreshed_preview: Option<Option<codec::Preview>>,
     result: Result<(), crate::io::SaveFailure>,
 }
-/// Which viewport background the interface colour picker is editing.
+/// Which Options colour the interface colour picker is editing.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
-pub enum BgTarget {
-    Model,
-    Paper,
-    Desk,
+pub enum UiColor {
+    ModelBackground,
+    PaperBackground,
+    DeskBackground,
+    /// One of the six theme colours, by its row in Options.
+    Theme(usize),
+    Crosshair,
+    SnapMarker,
+    CommandText,
 }
 
 /// Where a colour chosen in the standalone palette window should be applied.
@@ -1607,8 +1612,8 @@ pub enum ColorPickTarget {
     LayerState(usize),
     /// The MText editor's selection (or global) colour.
     MText,
-    /// An Options viewport background (interface colour, true colour only).
-    Background(BgTarget),
+    /// An Options interface colour (true colour only).
+    Interface(UiColor),
 }
 
 /// Table records the clipboard entities depend on, snapshotted from the source