Add WCAG 2.1 Level AA accessibility across all components

src/lib/components/Titlebar.svelte

@@ -9,8 +9,9 @@
   data-tauri-drag-region
   class="group absolute top-0 left-0 right-0 z-50 flex h-10 items-center justify-end pr-3.5 select-none"
 >
-  <!-- Centered app name -->
+  <!-- Centered app name (decorative — OS window title handles screen readers) -->
   <span
+    aria-hidden="true"
     class="pointer-events-none absolute inset-0 flex items-center justify-center
            text-[11px] font-semibold tracking-[0.3em] text-[#383838] uppercase"
     style="font-family: 'Space Mono', monospace;"
@@ -19,7 +20,7 @@
   </span>
 
   <!-- Traffic light buttons (order: maximize, minimize, close → close is rightmost) -->
-  <div class="flex items-center gap-[8px] opacity-10 transition-opacity duration-300 group-hover:opacity-100">
+  <div class="flex items-center gap-[8px] opacity-10 transition-opacity duration-300 group-hover:opacity-100 group-focus-within:opacity-100">
     <!-- Maximize (green) -->
     <button
       aria-label="Maximize"