feat(ui): scale interface for high resolution displays

docs/DESIGN_SYSTEM.md

@@ -26,6 +26,24 @@ The canonical tokens live in `src/app.css` under `:root`.
 | Main text | `--shd-text` | `#ecebe5` |
 | Muted text | `--shd-text-muted` | `#858b8d` |
 | SHD action orange | `--shd-orange` | `#f6a821` |
+| Adaptive display scale | `--shd-display-scale` | `1` by default; stepped on high-resolution desktops |
+
+### High-resolution display scaling
+
+The application uses restrained, stepped interface scaling when the effective
+CSS viewport is substantially wider than a conventional desktop. At 2200px the
+complete interface scales to 115%, at 2800px to 130%, and at 3400px to 150%.
+This preserves the established relationship between typography, controls,
+tables, spacing, hit targets, and fixed-position surfaces instead of enlarging
+text alone.
+
+Use the shared `--shd-display-scale` token and the root-level `zoom` treatment;
+do not add module-specific 4K font-size overrides. Responsive layouts below
+2200px remain unchanged. Browser zoom and operating-system display scaling still
+remain available to users and naturally change the effective CSS viewport.
+High-resolution scaling must not introduce horizontal overflow, clipped fixed
+surfaces, or a different semantic hierarchy. Test the standard desktop layout
+and each affected wide breakpoint after changing this policy.
 | Success | `--shd-success` | `#75c08c` |
 | Destructive/error | `--shd-danger` | `#df8069` |
 | Informational/defensive | `--shd-info` | `#70b9d6` |