feat(ui): Escape key to stop generation in Chat tab

Press Escape while generation is running to stop it (same as clicking Stop).
Also shows 'Esc' keyboard hint on the Stop button.

Changes:
- ui/src/ui/views/chat.ts: Add keydown handler for Escape, show kbd hint

ui/src/ui/views/chat.ts

@@ -240,7 +240,16 @@ export function renderChat(props: ChatProps) {
   `;
 
   return html`
-    <section class="card chat">
+    <section
+      class="card chat"
+      tabindex="-1"
+      @keydown=${(e: KeyboardEvent) => {
+        if (e.key === "Escape" && canAbort && props.onAbort) {
+          e.preventDefault();
+          props.onAbort();
+        }
+      }}
+    >
       ${props.disabledReason
         ? html`<div class="callout">${props.disabledReason}</div>`
         : nothing}
@@ -359,7 +368,9 @@ export function renderChat(props: ChatProps) {
               ?disabled=${!props.connected || (!canAbort && props.sending)}
               @click=${canAbort ? props.onAbort : props.onNewSession}
             >
-              ${canAbort ? "Stop" : "New session"}
+              ${canAbort ? "Stop" : "New session"}${canAbort
+                ? html`<kbd class="btn-kbd">Esc</kbd>`
+                : nothing}
             </button>
             <button
               class="btn primary"