🧪 Add tests for initWasm and extract inline script to app.js

[paul0728]

🎯 What: The initWasm function and related initialization logic were inline in docs/index.html and completely untested, leaving a critical part of the application (WASM engine initialization) without coverage.
📊 Coverage: The logic was refactored by moving it into a testable ES module (docs/app.js). Tests were added in docs/app.test.js to cover the initWasm function, specifically covering both the happy path (successful initialization and DOM update) and the error path (fetch failure and corresponding DOM error state).
✨ Result: Test coverage is greatly improved for the core WASM initialization. The application code is now modular and testable, establishing a solid pattern for future tests. The tests catch real bugs as verified locally.

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PR created automatically by Jules for task 6154815039559539078 started by @paul0728

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