Rename Text size to Interface; scale panel header with it

Action buttons, server picker, and its dropdown now follow the size
setting via new --action-h/--action-font/--action-icon variables.
Default tier matches the old fixed sizes; tab bar stays fixed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ui/src/app/tab-bar/tab-bar.ts

@@ -169,7 +169,7 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
           </div>
 
           <div class="mt-2.5 flex items-center justify-between gap-2">
-            <span class="text-xs text-muted-foreground">Text size</span>
+            <span class="text-xs text-muted-foreground">Interface</span>
             <div class="flex rounded-md border border-border p-0.5">
               @for (size of textSizes; track size) {
                 <button