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>

tests/ui/math.test.ts

@@ -22,7 +22,7 @@ describe('centreY', () => {
 });
 
 describe('formatNumber', () => {
-  it('always shows three decimals', () => {
+  it('shows three decimals by default', () => {
     expect(NUMBER_DECIMALS).toBe(3);
     expect(formatNumber(100)).toBe('100.000');
     expect(formatNumber(0.5)).toBe('0.500');
@@ -50,6 +50,15 @@ describe('formatNumber', () => {
     expect(formatNumber(1.25, 1)).toBe('1.3');
     expect(formatNumber(1.25, 0)).toBe('1');
   });
+
+  it('shows a field of integral values without a decimal point at all', () => {
+    // What `NumberFieldOptions.decimals: 0` buys: a pixel index or a count of tiles reads
+    // "33" and "4" instead of "33.000" and "4.000".
+    expect(formatNumber(33, 0)).toBe('33');
+    expect(formatNumber(-4, 0)).toBe('-4');
+    // The minus-zero rule holds at every precision, not just the default one.
+    expect(formatNumber(-0.4, 0)).toBe('0');
+  });
 });
 
 describe('snapToStep', () => {