Add #[cfg(test)] coverage for src/config.rs: default round-trip,
backward-compat default-fill (old configs missing newer #[serde(default)]
fields still load), notifications_enabled default_true wiring, NetworkMode
default/ALL/Display/round-trip, and unknown-field tolerance. Derives
PartialEq on AppConfig to support equality asserts. Tests-only; no
production behavior change.
Implemented by Gemini (junior implementer), reviewed and verified by senior
(cargo build + clippy --all-targets + cargo test all green).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>