fix(design): replace hardcoded values with design tokens

Audit found ~35 violations against the token system. Fixed:
- Hardcoded shadows in layout.css replaced with --shadow-sm/md
- 8 rgba colors extracted to new glass tokens (--color-glass-*)
- border-radius: 50% replaced with var(--radius-full)
- ~25 off-grid spacing values (5px, 6px, 7px, 14px, 15px, 22px,
  26px, 34px) aligned to 4px grid using space tokens

public/styles/tokens.css

@@ -131,6 +131,12 @@
   --color-overlay: rgba(0, 0, 0, 0.45);
   --color-overlay-light: rgba(0, 0, 0, 0.2);
 
+  /* Glass-Overlays (fuer Elemente auf farbigen Hintergruenden) */
+  --color-glass:       rgba(255, 255, 255, 0.18);
+  --color-glass-hover: rgba(255, 255, 255, 0.3);
+  --color-glass-border: rgba(255, 255, 255, 0.15);
+  --color-danger-translucent: rgba(255, 59, 48, 0.35);
+
   /* --------------------------------------------------------
    * 8. Schatten
    *    3 Stufen: subtle (Karten), medium (Dropdowns, Hover),