fix: remove body background-color/color transitions to prevent theme flash

- Remove transition: background-color/color from body in all CSS files
- These transitions caused visible flash when navigating between pages
- The browser would animate from old theme colors to new theme colors

pos/static/css/dashboard.css

@@ -23,8 +23,6 @@
       min-height: 100vh;
       display: flex;
       flex-direction: column;
-      transition: background-color var(--duration-normal) var(--ease-in-out),
-                  color var(--duration-normal) var(--ease-in-out);
     }
 
     /* Modern theme dot-grid on body */