fonts: bump text-xs (extra small) to text-sm size

Redefine .text-xs to render at 0.875rem/1.25rem (text-sm), covering the PWA
and both help pages via the shared stylesheet. Explicit leading-* overrides
still apply.

SW cache v41.

frontend/style.css

@@ -1325,8 +1325,8 @@ video {
 }
 
 .text-xs {
-  font-size: 0.75rem;
-  line-height: 1rem;
+  font-size: 0.875rem;
+  line-height: 1.25rem;
 }
 
 .font-bold {

frontend/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'otaman-v40';
+const CACHE = 'otaman-v41';
 const URLS = [
   'index.html',
   'help.html',