Add structure icon size graphics override (#4270)

## Summary

Adds a new **Structure icon size** option to `GraphicsOverrides`,
exposed as a slider in the Graphics Settings modal. Players can now
scale how large structure icons are drawn on the map.

## Changes

- **`GraphicsOverrides.ts`** — add `iconSize: z.number()` to the
`structure` override schema.
- **`RenderOverrides.ts`** — apply the override onto
`settings.structure.iconSize` (consumed by
`StructurePass`/`StructureLevelPass` shaders).
- **`GraphicsSettingsModal.ts`** — add a slider (range 20–120, step 5)
in the "Structure Icons" section, with getter/handler following the
existing pattern. Falls back to the `render-settings.json` default of 60
when unset.
- **`resources/lang/en.json`** — add `icon_size_label` /
`icon_size_desc` (English only, per i18n rules).
- **`tests/GraphicsOverrides.test.ts`** — schema-validation cases plus
application tests (override sets the value; absence keeps the default).

The setting persists via the existing `userSettings.graphicsOverrides()`
localStorage flow and takes effect live through the existing
`regenerateRenderSettings` wiring.

## Testing

- `npx vitest tests/GraphicsOverrides.test.ts --run` — 35 passed
- `tsc --noEmit` — no new type errors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/GraphicsOverrides.test.ts

@@ -41,6 +41,8 @@ describe("GraphicsOverridesSchema", () => {
       { structure: { classicNumbers: true } },
       { structure: { classicNumbers: false } },
       { structure: { classicIcons: true, classicNumbers: false } },
+      { structure: { iconSize: 80 } },
+      { structure: { iconSize: 40, classicIcons: false } },
       { name: { darkNames: true }, structure: { classicIcons: true } },
     ];
     for (const c of cases) {
@@ -262,6 +264,16 @@ describe("applyGraphicsOverrides", () => {
     expect(absent.iconAlpha).toBe(0.9);
   });
 
+  test("iconSize override sets structure.iconSize", () => {
+    expect(gen({ structure: { iconSize: 90 } }).structure.iconSize).toBe(90);
+  });
+
+  test("iconSize absent → keeps render-settings.json default", () => {
+    const def = createRenderSettings().structure.iconSize;
+    expect(gen({ structure: {} }).structure.iconSize).toBe(def);
+    expect(gen({}).structure.iconSize).toBe(def);
+  });
+
   test("classicNumbers=true → classic bitmap font", () => {
     expect(
       gen({ structure: { classicNumbers: true } }).structureLevel.classicFont,