feat(call): add streaming voice call experience

src/app/chat/components/chat-header.tsx

@@ -5,7 +5,7 @@
  * 顶部保持返回、首充优惠、收藏入口三段结构。
  */
 import type { ReactNode } from "react";
-import { Lock } from "lucide-react";
+import { Lock, Phone } from "lucide-react";
 
 import { BackButton } from "@/app/_components";
 import { FavoriteEntryButton } from "@/app/_components/core";
@@ -27,6 +27,7 @@ export function ChatHeader({
   const navigator = useAppNavigator();
   const character = useActiveCharacter();
   const characterRoutes = useActiveCharacterRoutes();
+  const voiceCallEnabled = process.env.NEXT_PUBLIC_ENABLE_VOICE_CALL === "true";
 
   return (
     <header className="flex shrink-0 flex-col items-stretch bg-transparent p-0">
@@ -58,10 +59,24 @@ export function ChatHeader({
 
         <div className="flex min-w-0 justify-center">{offerBanner}</div>
 
-        <FavoriteEntryButton
-          characterSlug={character.slug}
-          tone="dark"
-        />
+        <div className="flex items-center gap-2">
+          {voiceCallEnabled ? (
+            <button
+              type="button"
+              className="inline-flex size-(--responsive-icon-button-size,42px) cursor-pointer items-center justify-center rounded-full border border-[rgba(255,255,255,0.12)] bg-[rgba(13,11,20,0.7)] text-white shadow-[0_10px_24px_rgba(0,0,0,0.2)] backdrop-blur-md transition hover:bg-[rgba(28,24,39,0.82)] active:scale-96"
+              aria-label={`Call ${character.shortName}`}
+              data-analytics-key="chat.start_voice_call"
+              data-analytics-label="Start voice call"
+              onClick={() => navigator.push(characterRoutes.call)}
+            >
+              <Phone size={19} strokeWidth={2.3} aria-hidden="true" />
+            </button>
+          ) : null}
+          <FavoriteEntryButton
+            characterSlug={character.slug}
+            tone="dark"
+          />
+        </div>
       </div>
     </header>
   );