docs: use absolute GIF URL so the demo renders on PyPI too

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

Author: cursor-usage-cli

README.md

@@ -13,7 +13,7 @@
 </p>
 
 <p align="center">
-  <img src="docs/demo.gif" alt="cursor-usage demo — monthly summary and per-day breakdown" width="760">
+  <img src="https://raw.githubusercontent.com/javaisbetterthanpython/cursor-usage/main/docs/demo.gif" alt="cursor-usage demo — monthly summary and per-day breakdown" width="760">
 </p>
 
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