feat(readme): replace header banner with a clean logo lockup

Swap the wide hero banner for a compact mark + wordmark lockup, served via
<picture> so the wordmark flips light/dark with the GitHub theme (the colored
Ocean mark is theme-independent). Applies to EN + zh READMEs. Removes the
now-orphaned docs/images/logo.png banner copy (the docs site keeps its own).

Claude-Session: https://claude.ai/code/session_01XFiuAUxNS9riUJFFBEHvui

Author: Shutong Wu

README.md

@@ -1,4 +1,7 @@
-<img width="820" alt="MCP for Unity" src="docs/images/logo.png" />
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="docs/images/logo-header-dark.png">
+  <img alt="MCP for Unity" src="docs/images/logo-header-light.png" width="400">
+</picture>
 
 | [English](README.md) | [简体中文](docs/i18n/README-zh.md) |
 |----------------------|---------------------------------|

docs/i18n/README-zh.md

@@ -1,4 +1,7 @@
-<img width="676" height="380" alt="MCP for Unity" src="../images/logo.png" />
+<picture>
+  <source media="(prefers-color-scheme: dark)" srcset="../images/logo-header-dark.png">
+  <img alt="MCP for Unity" src="../images/logo-header-light.png" width="400">
+</picture>
 
 | [English](../../README.md) | [简体中文](README-zh.md) |
 |----------------------|---------------------------------|