fix(web): use existing STYLE_MAP key for home 「自动」 fallback

After bed4dc5 renamed style keys to include the (Image N参考) suffix,
the home start() still resolved 「自动」 against the legacy bare name
「京阿尼细腻日常」, leaving styleGuide undefined and tripping the
/api/start required-field check on the default click.

Fall back to "Galgame CG 梦幻光影" — a key that actually exists in
STYLE_MAP — so the default path resolves cleanly without changing the
behavior of explicitly selected styles.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/page.tsx

@@ -1057,14 +1057,15 @@ export default function HomePage() {
       .filter(Boolean)
       .join("\n");
 
-    // 「自动」→ fall back to 二次元 (project default). Plain prompts like
+    // 「自动」→ fall back to Galgame CG (project default). Plain prompts like
     // "由模型自动判断画风" are not understood by FLUX — it just paints them
     // literally, so we'd rather lock in a sensible default.
     // TODO(自动路由): 后续实现真正的「自动」——由模型依据世界观 / 玩家 prompt
-    // 选出最合适的画风，再映射到对应风格提示词，而非固定回退到二次元。届时
+    // 选出最合适的画风，再映射到对应风格提示词，而非固定回退到 Galgame。届时
     // 同步更新风格弹窗副标题（「由模型根据 prompt 判断风格」）使文案与行为一致。
-    const effectiveStyle = artStyle === "自动" ? "京阿尼细腻日常" : artStyle;
-    const styleGuide = STYLE_MAP[effectiveStyle] ?? STYLE_MAP["京阿尼细腻日常"]!;
+    const DEFAULT_STYLE = "Galgame CG 梦幻光影";
+    const effectiveStyle = artStyle === "自动" ? DEFAULT_STYLE : artStyle;
+    const styleGuide = STYLE_MAP[effectiveStyle] ?? STYLE_MAP[DEFAULT_STYLE]!;
     const audioEnabled = voice === "开启";
 
     sessionStorage.setItem(