Warn when server binary is built without web assets

[ptone]

Add a prominent startup warning when web assets are not embedded and no --web-assets-dir is provided. Serve the self-contained noAssetsPage HTML from the static asset handler instead of a plain text 404, so direct requests to /assets/* also show a helpful message.

Fixes #<issue_number_goes_here>

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to documentation are included in the PR

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[gemini-code-assist]

Code Review

This pull request adds a warning when the server is started without web assets and updates the static asset handler to serve a user-friendly HTML page instead of a plain text error when assets are missing. The reviewer suggested using structured logging (slog.Warn) instead of standard log.Printf to ensure warnings are correctly formatted in production environments.

[gemini-code-assist]

The application is configured to use structured logging via slog (which supports GCP logging format, OTel, etc.). Using standard log.Printf bypasses the structured logging handler, meaning these warnings won't be correctly formatted as structured JSON logs in production/hosted environments. Consider using slog.Warn instead to ensure the warning is properly captured by the configured logging pipeline.

Suggested change

	log.Printf("WARNING: This binary was built without web assets. The web UI will not be available.")
	log.Printf("Run 'make web' and rebuild to include the web frontend, or use --web-assets-dir.")
	slog.Warn("This binary was built without web assets. The web UI will not be available. Run 'make web' and rebuild to include the web frontend, or use --web-assets-dir.")