Add Classic Icons toggle to Graphics Settings

Adds a "Classic icons" toggle in the structure-icons section of the
Graphics Settings modal. Off (default) keeps today's renderer look;
on switches to a classic style — lighter player-colored shape behind
a dark icon glyph, with 0.75 alpha for a subtle translucent feel.

Exposes the underlying tuning as new render-settings knobs
(`structure.fillDarken`, `borderDarken`, `iconAlpha`, `iconR/G/B`) and
threads them through the structure shader as uniforms, replacing the
previously hardcoded `darken(_, 0.65)` / `darken(_, 0.35)` calls and
the hardcoded white `vec3(1.0)` icon color. The `classicIcons` boolean
in the override schema is the single user-facing knob; the generator
derives the five underlying field values from it. Extends the
ClientGameRunner live-apply path to copy the `structure` slice too,
and adds tests covering the schema and preset derivation.

src/client/hud/layers/GraphicsSettingsModal.ts

@@ -127,6 +127,25 @@ export class GraphicsSettingsModal extends LitElement implements Controller {
     this.requestUpdate();
   }
 
+  private patchStructure(patch: Partial<GraphicsOverrides["structure"]>) {
+    const current = this.userSettings.graphicsOverrides();
+    this.userSettings.setGraphicsOverrides({
+      ...current,
+      structure: { ...current.structure, ...patch },
+    });
+    this.requestUpdate();
+  }
+
+  private currentClassicIcons(): boolean {
+    return (
+      this.userSettings.graphicsOverrides().structure?.classicIcons ?? false
+    );
+  }
+
+  private onToggleClassicIcons() {
+    this.patchStructure({ classicIcons: !this.currentClassicIcons() });
+  }
+
   private onNameScaleChange(event: Event) {
     const value = parseFloat((event.target as HTMLInputElement).value);
     this.patchName({ nameScaleFactor: value });
@@ -159,6 +178,7 @@ export class GraphicsSettingsModal extends LitElement implements Controller {
     const nameScale = this.currentNameScale();
     const nameCull = this.currentNameCull();
     const namesColored = !this.currentDarkNames();
+    const classicIcons = this.currentClassicIcons();
 
     return html`
       <div
@@ -264,6 +284,31 @@ export class GraphicsSettingsModal extends LitElement implements Controller {
               </div>
             </button>
 
+            <div
+              class="px-3 py-1 text-xs font-semibold text-slate-400 uppercase tracking-wider mt-2"
+            >
+              ${translateText("graphics_setting.section_structure_icons")}
+            </div>
+
+            <button
+              class="flex gap-3 items-center w-full text-left p-3 hover:bg-slate-700 rounded-sm text-white transition-colors"
+              @click=${this.onToggleClassicIcons}
+            >
+              <div class="flex-1">
+                <div class="font-medium">
+                  ${translateText("graphics_setting.classic_icons_label")}
+                </div>
+                <div class="text-sm text-slate-400">
+                  ${translateText("graphics_setting.classic_icons_desc")}
+                </div>
+              </div>
+              <div class="text-sm text-slate-400">
+                ${classicIcons
+                  ? translateText("user_setting.on")
+                  : translateText("user_setting.off")}
+              </div>
+            </button>
+
             <div class="border-t border-slate-600 pt-3 mt-4">
               <button
                 class="flex gap-3 items-center w-full text-left p-3 hover:bg-slate-700 rounded-sm text-white transition-colors"