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>

resources/lang/en.json

@@ -535,6 +535,8 @@
     "hover_glow_alpha_label": "Hover glow strength",
     "hover_glow_width_desc": "How far the white glow extends behind the hovered player's name",
     "hover_glow_width_label": "Hover glow size",
+    "icon_size_desc": "How large structure icons are drawn on the map",
+    "icon_size_label": "Structure icon size",
     "name_cull_desc": "Hide names smaller than this size",
     "name_cull_label": "Minimum name size",
     "name_scale_label": "Name Scale",