fix: connect --app-font-size to --text-base token and remove redundant overrides

--app-font-size was still 100% (16px), disconnected from the type scale
tokens. Now references var(--text-base) so the 14px enterprise baseline
actually takes effect. Remove empty responsive @media font-size placeholders
in variables.base.css and raw heading size overrides in app-shell.css that
fought the clamp()-based tokens.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/css/layout/app-shell.css

@@ -366,39 +366,12 @@
     padding-inline: calc(var(--app-spacing) * 1);
   }
 
-  /* NOTE: These responsive heading overrides may be redundant now that
-     --text-2xl … --text-5xl use clamp() for fluid scaling. Kept pending
-     browser verification — removal is tracked separately. */
   @media (min-width: 576px) {
     .container {
       max-width: 510px;
       padding-right: 0;
       padding-left: 0;
     }
-
-    h1 {
-      --app-font-size: 1.7rem;
-    }
-
-    h2 {
-      --app-font-size: 1.6rem;
-    }
-
-    h3 {
-      --app-font-size: 1.3rem;
-    }
-
-    h4 {
-      --app-font-size: 1.2rem;
-    }
-
-    h5 {
-      --app-font-size: 1.1rem;
-    }
-
-    h6 {
-      --app-font-size: 1rem;
-    }
   }
 
   @media (min-width: 768px) {