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/login.css

@@ -19,8 +19,6 @@
       font-size: var(--text-body);
       color: var(--color-text-primary);
       background-color: var(--color-bg-base);
-      transition: background-color var(--duration-slow) var(--ease-in-out),
-                  color var(--duration-slow) var(--ease-in-out);
       min-height: 100vh;
     }
 
@@ -58,8 +56,6 @@
       background-color: var(--color-bg-elevated);
       border-bottom: 1px solid var(--color-border);
       box-shadow: var(--shadow-sm);
-      transition: background-color var(--duration-slow) var(--ease-in-out),
-                  border-color var(--duration-slow) var(--ease-in-out);
     }
 
     .theme-bar__label {
@@ -657,8 +653,6 @@
       justify-content: space-between;
       gap: var(--space-4);
       background: var(--color-surface-1);
-      transition: background-color var(--duration-slow) var(--ease-in-out),
-                  border-color var(--duration-slow) var(--ease-in-out);
     }
 
     .footer-version {