修复移动端软键盘页面弹跳黑底

移除 H5 软键盘打开时平台壳全局 transform 位移，避免浏览器原生避让后再次弹跳。

保留键盘打开状态、底部 dock 隐藏和浅色根背景兜底，避免短表单露出黑色宿主底色。

补充小程序 web-view 原生 page 浅色背景和对应样式测试。

更新统一创作页与平台键盘适配文档，沉淀不再全局上移平台壳的约束。

src/index.test.ts

@@ -59,6 +59,30 @@ describe('index stylesheet unread dots', () => {
     expect(css).toContain('::-webkit-scrollbar-thumb');
   });
 
+  it('uses the platform fill for root background exposed by mobile keyboard shift', () => {
+    const css = readIndexCss();
+
+    const keyboardRootBlock = getCssBlock(
+      css,
+      "html[data-mobile-keyboard-open='true'],\nhtml[data-mobile-keyboard-open='true'] body,\nhtml[data-mobile-keyboard-open='true'] #root",
+    );
+    expect(keyboardRootBlock).toContain('--platform-keyboard-exposed-fill');
+    expect(keyboardRootBlock).toContain('#fffdf9');
+    expect(keyboardRootBlock).not.toContain('#0a0a0a');
+  });
+
+  it('does not globally transform the platform shell while the mobile keyboard is open', () => {
+    const css = readIndexCss();
+
+    const platformShellBlock = getCssBlock(
+      css,
+      '.platform-viewport-shell {\n  height',
+    );
+    expect(platformShellBlock).toContain('--platform-layout-viewport-height');
+    expect(platformShellBlock).not.toContain('translate3d');
+    expect(platformShellBlock).not.toContain('--platform-keyboard-focus-offset');
+  });
+
   it('uses warm brown tokens for draft unread markers instead of red literals', () => {
     const css = readIndexCss();