feat: yoloVisibility config, alwaysShowLabel on groups, site settings panel

- AppConfig gains yoloVisibility: 'auto' | 'never' (default: auto)
- Group gains alwaysShowLabel?: boolean
- YOLO toggle only renders when showYoloToggle is true (auto + >1 group)
- Group divider shows when >1 group OR alwaysShowLabel is set
- New /admin/api/config GET+PUT endpoint for yolo settings
- Admin: alwaysShowLabel checkbox in group form
- Admin: ⚙ Settings panel with YOLO label, description, and visibility controls

src/lib/server/data.ts

@@ -15,6 +15,12 @@ export interface AppConfig {
   yoloLabel?: string;
   /** Description shown inline after the label. Defaults to "(cross-group conversions)". */
   yoloDescription?: string;
+  /**
+   * Controls when the YOLO mode toggle is visible to users.
+   * 'auto'  — show only when more than one group is present (default)
+   * 'never' — always hidden
+   */
+  yoloVisibility?: 'auto' | 'never';
 }
 
 /** Read units.json synchronously. Returns parsed UnitsData. */