Add a decimals option, fix dropdown overflow and the texture filter

- NumberField takes a decimals option (default 3); frame rects and the
  slice dialog use 0, matching their integer model.
- Button truncates its label to the drawn width; Dropdown reserves the
  chevron slot. NodeView reuses the shared truncate().
- Setting scaleMode leaves the style's shared resource id in place, so
  the old GPU sampler stayed in use until a reload. Call style.update().
- Graph gets a header bar like the Texture tab: GraphEditor is now the
  frame, the former editor is GraphCanvas. A selection the tab does not
  concern no longer unloads the behavior.
- New e2e step 14 guards text overflow across all widgets.

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

src/ui/widgets/Dropdown.ts

@@ -24,6 +24,11 @@ export class Dropdown extends Button {
     return { w: size.w + CHEVRON_SLOT, h: size.h };
   }
 
+  /** The chevron's room is not the label's: a long value is shortened, not drawn under it. */
+  protected override labelSlot(): number {
+    return super.labelSlot() - CHEVRON_SLOT;
+  }
+
   override redraw(): void {
     super.redraw();
     const x = this.w - CHEVRON_SLOT / 2 - Theme.padding / 2;