Improved error handling, and notices about device limitations + HTTPS warning

[supermoos]

Improve setup instructions and add runtime warnings

This commit enhances the user and developer experience for the Emoji Gemma web demos by providing clearer setup instructions and adding important warnings about runtime requirements.

Key changes include:

• Updated READMEs with instructions for setting up an HTTPS server, which is necessary for WebGPU on some platforms (e.g., iOS Safari).
• Added a "Device Limitations" section to the READMEs to warn users about high memory usage and potential crashes on mobile devices.
• Improved the WebGPU availability check in the app-mediapipe demo to detect and notify users when running in a non-secure context.

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[gemini-code-assist]

Summary of Changes

Hello @supermoos, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the user experience for the Emoji Gemma web demos by providing clearer setup instructions, adding warnings about device limitations and HTTPS requirements, and improving error handling for WebGPU availability. These changes make the demos more accessible and robust across different platforms and devices.

Highlights

• Improved Setup Instructions: The README files for both the app-mediapipe and app-transformersjs demos have been updated with more detailed instructions for setting up a local web server, including guidance on creating a secure (HTTPS) server, which is required for WebGPU on some platforms like iOS Safari.
• Device Limitations Warning: A new "Device Limitations" section has been added to the README files to warn users about the high memory usage of language models and the potential for crashes on memory-constrained devices, especially iOS devices.
• WebGPU Availability Check: The app-mediapipe demo now includes an improved WebGPU availability check that detects and notifies users when running in a non-secure context (HTTP), advising them to use HTTPS.
• Preload Worker Resource: The index.html file in app-mediapipe has been updated to properly preload the worker script.

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[gemini-code-assist]

Code Review

This pull request significantly improves the user experience by adding detailed setup instructions for running a secure HTTPS server, which is a requirement for WebGPU on some platforms. It also adds important warnings about device memory limitations and enhances the WebGPU availability check to inform users about running in a non-secure context. The changes are clear and valuable. I've suggested a couple of minor improvements to the documentation and fixed a typo in a warning message.

[gemini-code-assist]

The openssl command includes a very specific subject. While this works, a simpler subject focusing only on the Common Name (CN) is sufficient for a local development certificate and is less confusing for users who might think they need to change these values.

Suggested change

	```bash
	openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pem -sha256 -days 365 -nodes -subj "/C=US/ST=CA/L=Mountain View/O=Dev/CN=localhost"
	```
	```bash
	openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pem -sha256 -days 365 -nodes -subj "/CN=localhost"
	```

[gemini-code-assist]

There appears to be a typo "iOS26" in the note. It's better to either correct it to a specific version or make it more general, like 'iOS Safari', to avoid it becoming outdated.

Suggested change

	message += `
	<br/><br/>
	<strong>Note:</strong> WebGPU requires a secure context. Please access this page over HTTPS or from 'localhost'. Accessing a local server via its IP address also requires an HTTPS setup in many instances (e.g. iOS26 Safari).
	`;
	message += `
	<br/><br/>
	<strong>Note:</strong> WebGPU requires a secure context. Please access this page over HTTPS or from 'localhost'. Accessing a local server via its IP address also requires an HTTPS setup in many instances (e.g. iOS Safari).
	`;

[gemini-code-assist]

The openssl command includes a very specific subject. While this works, a simpler subject focusing only on the Common Name (CN) is sufficient for a local development certificate and is less confusing for users who might think they need to change these values.

Suggested change

	```bash
	openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pem -sha256 -days 365 -nodes -subj "/C=US/ST=CA/L=Mountain View/O=Dev/CN=localhost"
	```
	```bash
	openssl req -x509 -newkey rsa:2048 -keyout key.pem -out cert.pem -sha256 -days 365 -nodes -subj "/CN=localhost"
	```