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/math.ts

@@ -8,13 +8,14 @@ export function centreY(containerHeight: number, itemHeight: number): number {
   return Math.round((containerHeight - itemHeight) / 2);
 }
 
-/** Decimal places every number field shows. */
+/** Decimal places a number field shows unless it asks for its own (`NumberFieldOptions.decimals`). */
 export const NUMBER_DECIMALS = 3;
 
 /**
  * Display text for a number. Always shows `decimals` places, so a value can never *look* like
  * a whole number when it is not: precision used to follow the field's step, which hid the
- * decimals that dragging in the viewport or a graph node had put into the value.
+ * decimals that dragging in the viewport or a graph node had put into the value. Fields whose
+ * value is integral by nature (a pixel index, a count) pass 0 and are exact all the same.
  * A value that rounds to zero never carries a minus sign.
  */
 export function formatNumber(value: number, decimals = NUMBER_DECIMALS): string {