feat(ui): migrate style editors + nested colour picker (Plan B)

Move Table Style, MLeader Style and Dimension Style editors off OS
windows onto `active_modal` / `modal_content`, and turn the colour
picker into a nested modal that stacks over whichever dialog requested
it (`color_pick_target.is_some()` drives the overlay; CloseColorPicker
dismisses it). OpenColorWindow/ColorWindowPick no longer open/close an
OS window.

All thirteen former pop-up windows now render as in-canvas modals on
both native and web; only the native-only assoc/unsaved/save dialogs
remain as OS windows. Native + web build; 75 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -1321,6 +1321,8 @@ pub enum Message {
     DsColorMore(DsField),
     /// Open the standalone palette window targeting the given field.
     OpenColorWindow(ColorPickTarget),
+    /// Close the nested colour-picker modal without choosing (Plan B).
+    CloseColorPicker,
     /// A colour was chosen in the standalone palette window.
     ColorWindowPick(acadrust::types::Color),
     /// Set a block/linetype Handle field on the selected dim style from a