Improve config load observability and error reporting

[JanSmrcka]

Problem

Config parse/load failures are partially swallowed, reducing debuggability as config surface grows.

Scope

• Make config load errors explicit and user-friendly
• Distinguish missing config vs invalid config
• Add tests for malformed config handling

Acceptance Criteria

• Invalid config yields clear error/warning path
• Missing config keeps graceful defaults
• Config failure paths covered by tests

Why

Clear config diagnostics lower support/debug cost over time.