Fix GitHub Pages asset loading

[G-Fourteen]

Summary

• dynamically resolve the directory URL before injecting the stylesheet and script so GitHub Pages serves the assets when the trailing slash is missing
• share the same path resolution in app.js so the system prompt fetch succeeds regardless of how the page URL is formatted

Testing

• python tests/run_tests.py

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https://chatgpt.com/codex/tasks/task_b_69054ca85e688329b537e8e37911d1d6