feat: 媒体流图片自适应显示(单图/2图/Telegram式相册)

- 单图气泡按真实比例显示:横图限高260px、过高裁上下;竖图完整铺满宽度不裁、无黑边
- 2张同类相册:都竖图左右并排、都横图上下堆叠,按比例不裁
- 3+张相册:Telegram式马赛克拼贴(竖主图占左+其余堆右 / 横主图占顶+其余排底)
- 图片比例优先用后端width/height,缺失时从加载后的naturalWidth/Height读取
- 新增 constants/media.ts 统一尺寸规范;albumLayout 纯算法附单测

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/messageStream/hooks/useImageRatios.ts

@@ -0,0 +1,47 @@
+import { useEffect, useState } from "react";
+import type { Attachment } from "../../../types/post";
+
+/**
+ * Resolves each attachment's aspect ratio (width / height). Seeds from
+ * backend-provided width/height when present (no flash), then refines by
+ * loading the image and reading its intrinsic size — so it works even when the
+ * backend omits dimensions. Entries are `undefined` until known.
+ */
+export function useImageRatios(
+  attachments: Attachment[],
+  sources: (string | undefined)[],
+): (number | undefined)[] {
+  const seed = attachments.map((a) =>
+    a.width && a.height ? a.width / a.height : undefined,
+  );
+  const [ratios, setRatios] = useState<(number | undefined)[]>(seed);
+
+  const key = sources.join("|");
+  useEffect(() => {
+    let cancelled = false;
+    setRatios(
+      attachments.map((a) =>
+        a.width && a.height ? a.width / a.height : undefined,
+      ),
+    );
+    sources.forEach((src, i) => {
+      if (!src) return;
+      const img = new Image();
+      img.onload = () => {
+        if (cancelled || !img.naturalWidth || !img.naturalHeight) return;
+        setRatios((prev) => {
+          const next = prev.slice();
+          next[i] = img.naturalWidth / img.naturalHeight;
+          return next;
+        });
+      };
+      img.src = src;
+    });
+    return () => {
+      cancelled = true;
+    };
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [key]);
+
+  return ratios;
+}