fix(fe): light mode white text — remove background/color from body, set on .app-shell

body is outside the [data-theme] scope, so var(--text-primary)
on body always resolved to the dark :root value (#e8edf5, near-white).
Elements inside [data-theme] inherited this white color in light mode,
causing white-on-white text. Fix by moving background/color to
.app-shell which is inside the theme scope.

services/frontend/frontend/src/app.css

@@ -123,8 +123,6 @@ html {
 
 body {
   font-family: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
-  background: var(--surface-base);
-  color: var(--text-primary);
   line-height: 1.6;
   min-height: 100vh;
   overflow-x: hidden;
@@ -587,7 +585,8 @@ svg { display: inline-block; vertical-align: middle; }
   display: flex;
   flex-direction: column;
   height: 100vh;
-  background: color-mix(in srgb, var(--surface-base) 94%, transparent);
+  background: var(--surface-base);
+  color: var(--text-primary);
 }
 
 .app-header {