feat(accessibility): harden chat and keyboard interactions

Stream capability-authorized chat reads with cancellation, virtualize viewer rendering, centralize modal focus management, and add keyboard-accessible queue and VOD menus. Expose command palette combobox state and keep the document language synchronized with the selected locale.

src/styles.css

@@ -4914,6 +4914,7 @@ input[type="number"]::-webkit-outer-spin-button {
    per-message colour for the username (driven by Twitch's IRC color
    metadata). */
 .chat-viewer-row {
+    min-height: 29px;
     padding: 4px 8px;
     line-height: 1.55;
     border-radius: 4px;
@@ -4922,6 +4923,17 @@ input[type="number"]::-webkit-outer-spin-button {
     word-wrap: break-word;
 }
 
+.chat-viewer-virtual-canvas {
+    position: relative;
+    min-height: 100%;
+}
+
+.chat-viewer-virtual-rows {
+    position: absolute;
+    inset: 0 0 auto;
+    width: 100%;
+}
+
 .chat-viewer-row:hover {
     background: rgba(255, 255, 255, 0.04);
 }
@@ -5615,6 +5627,11 @@ input[type="number"]::-webkit-outer-spin-button {
 }
 
 .context-menu-item {
+    display: block;
+    width: 100%;
+    border: 0;
+    background: transparent;
+    text-align: left;
     padding: 8px 12px;
     cursor: pointer;
     font-size: 13px;
@@ -5627,6 +5644,11 @@ input[type="number"]::-webkit-outer-spin-button {
     background: rgba(145, 70, 255, 0.15);
 }
 
+.context-menu-item:focus-visible {
+    outline: 2px solid var(--accent);
+    outline-offset: -2px;
+}
+
 .context-menu-item.disabled {
     color: var(--text-secondary);
     opacity: 0.55;