fix(frontend): raise dark contrast, expand hit areas, and surface input limits

Dark muted surfaces missed WCAG AA; action and tertiary tokens plus a 32-pair contract close that. Hit targets, remaining-count, and Enter-to-send follow the interaction audit without changing visual sizes.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/error.tsx

@@ -76,7 +76,7 @@ export default function RootError({
             --bp-ink: #f2f0ea;
             --bp-ink-soft: #b3afa4;
             --bp-border: #3c3b37;
-            --bp-action: #d4785a;
+            --bp-action: #d78064;
             --bp-danger: #e08573;
           }
         }
@@ -86,7 +86,7 @@ export default function RootError({
           --bp-ink: #f2f0ea;
           --bp-ink-soft: #b3afa4;
           --bp-border: #3c3b37;
-          --bp-action: #d4785a;
+          --bp-action: #d78064;
           --bp-danger: #e08573;
         }
         .root-error-action:focus-visible { outline: 3px solid ${focus}; outline-offset: 2px; }