The editor ships its own font, Source Sans 3 and Code Pro (#202)

Theme named Inter and JetBrains Mono, but neither was ever shipped,
so every machine drew the editor in whatever it had installed. The
e2e runs measured text in a different face than the user saw.

- Five Latin woff2 faces live in src/ui/fonts/ next to their OFL
  licences: Source Sans 3 400/500/600/700 and Source Code Pro 400.
- fonts.ts loads them through FontFace, and main.ts waits for them
  before the first widget exists, so Pixi measures the real face.
- A face that fails to load is reported in the console and the
  system fallback in Theme takes over.
- Theme is retuned by hand: 12 px body, 22 px rows, smaller gaps
  and padding, radius 8, and weight 500 for bold and headings.
- The scene background default follows Theme.colors.graphBackground.
  This makes core import from ui for the first time.

Findings:
- The 500 file is in the folder but fonts.ts does not register it
  yet, so weight 500 currently falls back to the 400 face.

Tests:
- ./check.sh: 141 files, 2080 tests, exit 0.
- e2e not run. Checks that measure widths or row positions are
  expected to need updating for the new metrics.

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

src/core/properties/PropertySchema.ts

@@ -64,6 +64,7 @@ import {
 } from './particles';
 import { SHADER_KEY, shaderOf, shaderParamDefs } from './shaders';
 import { LOOP_KEY as SOUND_LOOP_KEY, VOLUME_KEY as SOUND_VOLUME_KEY } from './sounds';
+import { Theme } from '../../ui';
 
 /**
  * What a kind's definition carries beyond key, label and default. `decimals` belongs only on
@@ -250,7 +251,7 @@ export function isValidVariableName(name: string): boolean {
 /** Position is deliberately absent: it belongs to scene objects, not to asset types. */
 const ASSET_SCHEMAS: Record<AssetType, PropertySchema> = {
   scene: [
-    { key: 'backgroundColor', label: 'Background', kind: 'color', default: 0x2a2d34 },
+    { key: 'backgroundColor', label: 'Background', kind: 'color', default: Theme.colors.graphBackground },
     // Pulls on every dynamic rigidbody, in world units per second squared: 980 is Earth at 100 px = 1 m.
     { key: 'gravityX', label: 'Gravity X', kind: 'number', default: 0, step: 10 },
     { key: 'gravityY', label: 'Gravity Y', kind: 'number', default: 980, step: 10 },