Share the colour editor, add an eyedropper, use library code

- ColorEditor: the HSB / RGB / field editor and hex box, now shared by
  the Select Color panel and the Options background swatches. Those
  swatches open the panel without tabs or index colours, which removes
  the iced_aw color_picker feature and the BgPicker messages.
- Sliders are iced's built-in slider and vertical_slider over a
  gradient container; the custom GradientSlider widget is gone.
- Nearest index colour comes from opencadcodec's aci_table::nearest_aci.
- Eyedropper: a pipette at the right of the mode row hides the panel,
  captures the window with iced's screenshot task and shows an 11x11
  magnifier beside a crosshair; a click picks the pixel, Esc backs out.
  Not offered on the web build.
- The hex box applies six-digit values while typing.

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

src/app/mod.rs

@@ -838,9 +838,6 @@ pub(super) struct OpenCADStudio {
     pub(crate) sheet_set_pending_layout: Option<(String, String)>,
     /// The subset / set Import Layout as Sheet adds to, while its file picker is open.
     pub(crate) sheet_set_import_parent: Option<String>,
-    /// Which viewport background the colour wheel is editing, or `None` when
-    /// it is closed. One slot, because only one wheel can be open at a time.
-    pub(crate) bg_picker: Option<BgTarget>,
     /// General edge-stack dock layout for the side panels.
     pub(crate) dock: crate::ui::dock::DockState,
     /// Which panel is currently floated at full height (hovered, or a pinned
@@ -1580,7 +1577,7 @@ pub struct SaveOutcome {
     refreshed_preview: Option<Option<codec::Preview>>,
     result: Result<(), crate::io::SaveFailure>,
 }
-/// Which viewport background a colour-wheel session is editing.
+/// Which viewport background the interface colour picker is editing.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum BgTarget {
     Model,
@@ -1610,6 +1607,8 @@ 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),
 }
 
 /// Table records the clipboard entities depend on, snapshotted from the source
@@ -2311,12 +2310,6 @@ pub enum Message {
     ModelSpaceModeChanged(config::ModelSpaceMode),
     /// Change Model Space custom background color as hex or empty for default.
     ModelSpaceBgChanged(String),
-    /// Open the colour wheel on one of the viewport backgrounds.
-    BgPickerOpen(BgTarget),
-    /// Dismiss the wheel, changing nothing.
-    BgPickerCancel,
-    /// Accept the wheel's colour for whichever background it was opened on.
-    BgPickerSubmit(iced::Color),
     /// Change Paper Space custom sheet background color as hex or empty for default.
     PaperSpaceBgChanged(String),
     /// Change Paper Space desk surround background (#RRGGBB).
@@ -4283,7 +4276,6 @@ impl OpenCADStudio {
             sheet_set: Default::default(),
             sheet_set_pending_layout: None,
             sheet_set_import_parent: None,
-            bg_picker: None,
             block_palette: Default::default(),
             xref_manager: Default::default(),
             dock: Default::default(),