fix(ui): float the colour-selector popup instead of pushing the form (#94)

The colour list/palette was laid out inline, shifting everything below it
down when opened. Wrap it in a small custom widget (Floating) that renders
the button inline and the popup as an iced overlay anchored below it, so it
floats over the UI like a real dropdown. Picking a colour now also closes
the popup (the style editors clear their open-field state on edit).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update.rs

@@ -6544,6 +6544,7 @@ impl OpenCADStudio {
                 Task::none()
             }
             Message::TableStyleCellEdit { row, field, value } => {
+                self.ts_color_open = None;
                 let r = row as usize;
                 if r < 3 {
                     match field {
@@ -6950,6 +6951,7 @@ impl OpenCADStudio {
                 Task::none()
             }
             Message::MLeaderStyleEdit { field, value } => {
+                self.mls_color_open = None;
                 match field {
                     "landing_distance" => self.mls_landing_distance = value,
                     "landing_gap" => self.mls_landing_gap = value,
@@ -7304,6 +7306,7 @@ impl OpenCADStudio {
             }
             Message::DsEdit(field, val) => {
                 self.apply_ds_edit(field, val);
+                self.ds_color_open = None;
                 Task::none()
             }
             Message::DsToggle(field) => {