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/utils/albumLayout.test.ts

@@ -0,0 +1,54 @@
+import { describe, expect, it } from "vitest";
+import { computeAlbumLayout } from "./albumLayout";
+
+describe("computeAlbumLayout", () => {
+  it("returns null for fewer than 2 images", () => {
+    expect(computeAlbumLayout([])).toBeNull();
+    expect(computeAlbumLayout([1.5])).toBeNull();
+  });
+
+  it("places two portraits side by side (1 row, 2 cols)", () => {
+    const layout = computeAlbumLayout([0.7, 0.8])!;
+    expect(layout.gridTemplateRows).toBe("1fr");
+    expect(layout.tiles).toHaveLength(2);
+    // Both tiles occupy the single row, adjacent columns.
+    expect(layout.tiles[0]).toMatchObject({ colStart: 1, colEnd: 2 });
+    expect(layout.tiles[1]).toMatchObject({ colStart: 2, colEnd: 3 });
+    // Album width = sum of ratios when height = 1.
+    expect(layout.aspectRatio).toBeCloseTo(0.7 + 0.8, 4);
+  });
+
+  it("stacks two landscapes vertically (2 rows, 1 col)", () => {
+    const layout = computeAlbumLayout([1.6, 1.4])!;
+    expect(layout.gridTemplateColumns).toBe("1fr");
+    expect(layout.tiles[0]).toMatchObject({ rowStart: 1, rowEnd: 2 });
+    expect(layout.tiles[1]).toMatchObject({ rowStart: 2, rowEnd: 3 });
+    expect(layout.aspectRatio).toBeCloseTo(1 / (1 / 1.6 + 1 / 1.4), 4);
+  });
+
+  it("puts a portrait primary on the left with the rest stacked right", () => {
+    const layout = computeAlbumLayout([0.6, 1.2, 1.3])!;
+    // Primary spans both right-hand rows in column 1.
+    expect(layout.tiles[0]).toEqual({
+      colStart: 1,
+      colEnd: 2,
+      rowStart: 1,
+      rowEnd: 3,
+    });
+    expect(layout.tiles[1]).toMatchObject({ colStart: 2, rowStart: 1 });
+    expect(layout.tiles[2]).toMatchObject({ colStart: 2, rowStart: 2 });
+  });
+
+  it("puts a landscape primary on top with the rest in a row below", () => {
+    const layout = computeAlbumLayout([1.8, 0.9, 1.1])!;
+    // Primary spans both bottom columns in row 1.
+    expect(layout.tiles[0]).toEqual({
+      colStart: 1,
+      colEnd: 3,
+      rowStart: 1,
+      rowEnd: 2,
+    });
+    expect(layout.tiles[1]).toMatchObject({ rowStart: 2, colStart: 1 });
+    expect(layout.tiles[2]).toMatchObject({ rowStart: 2, colStart: 2 });
+  });
+});