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/BubbleImage.tsx

@@ -11,6 +11,12 @@ type BubbleImageProps = {
   fallbackSrc?: string | (string | undefined)[];
   className?: string;
   loading?: "lazy" | "eager";
+  /**
+   * Called once the active source loads, with the image's intrinsic pixel
+   * size. Lets callers (e.g. single-image bubbles) adopt the real aspect
+   * ratio without depending on backend-provided width/height.
+   */
+  onNaturalSize?: (width: number, height: number) => void;
 };
 
 /**
@@ -24,6 +30,7 @@ export function BubbleImage({
   fallbackSrc,
   className,
   loading,
+  onNaturalSize,
 }: BubbleImageProps) {
   // Ordered, de-duplicated list of sources to attempt in turn.
   const candidates = useMemo(() => {
@@ -62,6 +69,11 @@ export function BubbleImage({
       alt=""
       loading={loading}
       className={className}
+      onLoad={(e) => {
+        const img = e.currentTarget;
+        if (img.naturalWidth && img.naturalHeight)
+          onNaturalSize?.(img.naturalWidth, img.naturalHeight);
+      }}
       onError={() => setAttempt((i) => i + 1)}
     />
   );