Architecture: Support custom user-defined themes via JSON

[sfloess]

Part of #233: Architecture: Unify curses-java and curses-themes with shared implementation

Scope

Once themes are defined in JSON, extend the system to allow users to create and load custom themes without writing Java or Python code.

Tasks

• Define a theme search path (e.g., built-in resources, then ~/.config/curses-themes/, then ./themes/)
• Implement theme discovery: scan directories for .json theme files
• Validate user-provided themes against the JSON schema at load time with clear error messages
• Provide a CLI or API to list available themes
• Write documentation for theme authors: required fields, color values, border character format
• Add 1-2 example custom themes in documentation

Acceptance Criteria

• Users can drop a .json file into a themes directory and have it discovered
• Invalid themes produce clear validation errors (not stack traces)
• Built-in themes take precedence over user themes with same name (or vice versa, document the policy)
• Documentation sufficient for a user to create a new theme without reading source code