Rename the internal backplate vocabulary to root plate (cce-ui RFC 7a-4)

Mechanical rename, no behaviour change: identifiers that carried the
retired word (statics, registry slots, setters, trait methods, locals)
now say root_plate, and comment prose says root plate.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

 src/main.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/main.rs b/src/main.rs
index fdd7483..16657d4 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -419,7 +419,7 @@ impl Application for ListApp {
     }
 
     /// Drag the window by its title band; everywhere else is content.
-    fn is_movable_backplate_at(&self, _px: f32, py: f32) -> bool {
+    fn is_movable_root_plate_at(&self, _px: f32, py: f32) -> bool {
         py <= metrics(self.width as f32).band_h
     }