fix(annotative): stage scale-manager edits like the style managers

The scale manager now runs as a transaction, matching the style managers'
staging: New starts a blank editor draft (nothing is added to the drawing
until Apply), Delete removes immediately but staged, and closing the window
without Apply restores the snapshot taken on open — so a scale created or
deleted mid-session is discarded on cancel. Apply commits the staged list
changes as a single undo entry. This replaces the earlier behaviour where
New copied the selected scale and edits landed immediately.

A `changed` flag guards the commit so a no-op / failed Apply (blank or
invalid fields, or an edit that didn't apply) no longer dirties the drawing
or pushes a phantom undo entry. Set Current is deliberately outside the
transaction — it takes effect immediately like the scale pill (and syncs
CANNOSCALEVALUE), so it is never rolled back on close. Renaming the current
scale follows the pointer; editing one of the built-in fallback scales (shown
when the drawing has no scale list) materialises it as a real scale; the
snapshot restore touches only the ACAD_SCALELIST root entry.

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

src/ui/style/scale_manager.rs

@@ -146,28 +146,29 @@ pub fn view_window<'a, 'b>(
     });
 
     // ── Right: editor (name + paper:drawing) ──────────────────────────────
-    let field = |label: &'static str, value: &str, on: fn(String) -> Message| {
-        row![
-            text(label).size(11).color(DIM).width(96),
-            text_input("", value)
-                .on_input(on)
-                .style(field_style)
-                .size(12)
-                .padding([5, 8])
-                .width(Fill),
-        ]
-        .align_y(iced::Center)
-        .spacing(6)
-    };
+    let field =
+        |label: &'static str, ph: &'static str, value: &str, on: fn(String) -> Message| {
+            row![
+                text(label).size(11).color(DIM).width(96),
+                text_input(ph, value)
+                    .on_input(on)
+                    .style(field_style)
+                    .size(12)
+                    .padding([5, 8])
+                    .width(Fill),
+            ]
+            .align_y(iced::Center)
+            .spacing(6)
+        };
 
     let editor = container(
         column![
             text("Scale").size(10).color(DIM),
-            field("Name", name_buf, Message::ScaleManagerNameBuf),
-            field("Paper units", paper_buf, Message::ScaleManagerPaperBuf),
-            field("Drawing units", drawing_buf, Message::ScaleManagerDrawingBuf),
+            field("Name", "e.g. 1:50", name_buf, Message::ScaleManagerNameBuf),
+            field("Paper units", "1", paper_buf, Message::ScaleManagerPaperBuf),
+            field("Drawing units", "50", drawing_buf, Message::ScaleManagerDrawingBuf),
             Space::new().height(6),
-            text("Type the ratio in the fields, then New to add or Apply to update the selected scale.")
+            text("New starts a blank scale; edit the fields and click Apply to save it. Changes are discarded if you close without Apply.")
                 .size(10)
                 .color(DIM),
         ]