fix(hatch): pattern-scale edits take effect + background on/off

Property edits on a hatch were invisible: fills render from the
prebuilt model cache, which move/copy refreshed but the properties
path never did — so a pattern-scale change (which must also rescale
the stored final pattern lines by the ratio) changed nothing on
screen (#415). invalidate_property_targets now rebuilds the edited
fill's cached model via Scene::refresh_fill_model.

The background row gains an on/off box: off removes the
HATCHBACKGROUNDCOLOR record entirely (the colour could never be
reverted before), on seeds ByLayer. The codec covers the full
colour-method range (ByLayer / ByBlock / RGB / ACI) and the backdrop
resolves ByLayer / ByBlock through the style chain. "More Colors…" on
the background picker now opens the palette window targeting the
background colour — it used to just close the picker.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/mod.rs

@@ -831,6 +831,8 @@ pub enum ColorPickTarget {
     Table(u8, &'static str),
     /// Selected entities' colour (left properties panel).
     Properties,
+    /// Selected entities' background colour (hatch / MTEXT background row).
+    PropertiesBg,
     /// Current creation colour (ribbon).
     Ribbon,
     /// A layer's colour, by panel row index.