feat(config): warn on unknown local.yaml keys

[ryannzander]

Summary

The config loader silently dropped any unknown key in local.yaml. The
dataclass merge (_merge_dataclass: if not hasattr: continue) means a
typo like tx_powr_dbm instead of tx_power_dbm, a stray nested key, or a
whole misspelled section (transmt:) just does nothing — with no signal to
the operator. _apply_yaml now collects every unapplied key (as dotted
paths, recursing only into nested dataclasses so user-populated maps like
meshtastic.channel_keys aren't false-flagged) and logs a single WARNING
listing them plus the file path. A non-mapping top-level YAML is now warned
and skipped instead of relying on incidental no-crash behaviour.

Adds tests/test_config_loader.py — first direct coverage of the loader.

Why

Recurring "I set it in local.yaml but nothing happened" footgun. A
misspelled field gives zero feedback today, so operators burn time
debugging a setting that was never applied. Surfacing it as one log line
turns a silent failure into an obvious one. This is a pure config-validation

• logging change, which CONTRIBUTING lists as a safe area to contribute.

Type

• Bug fix
• Feature
• Docs
• Refactor
• UI
• Installer
• Region support
• Hardware change

Testing

python -m pytest tests/test_config_loader.py tests/test_radio_frequency_resolver.py
→ 24 passed. ruff check src/config.py tests/test_config_loader.py → clean.
New tests cover: flat/nested unknown keys, dotted nested paths, the
mapping-field exemption, an unknown top-level section, the non-mapping
guard, and a load_config integration case. Behaviour is otherwise
unchanged — unknown keys are still ignored, known keys still merge as before.

• Local only
• Tested on hardware
• Dashboard tested
• Docs only
• UI only

Hardware:

• Pi: n/a (config-layer change, no hardware exercised)
• Concentrator: n/a
• Node/radio: n/a
• Region: n/a
• OS: developed on Windows; logic is pure Python/stdlib + PyYAML, CI runs Python 3.12

Impact

Does this affect:

• Parsing
• Relay
• TX
• Radio driver
• Region logic
• UI only
• Installer only

Notes: None of the above. Touches only src/config.py (YAML→dataclass merge)
and adds a test. No RF/radio/concentrator/region code changed; the only new
runtime behaviour is a WARNING log line. Known keys are applied exactly as
before.

AI-assisted?

• No
• Yes

If yes, how: Code written by author of commit and PR, reviewed and tested by Claude Code (Anthropic).

[KMX415]

Rebased onto \eat/v0.7.6, CI green, merged. Ships with the v0.7.6 sprint (not a separate patch). Thanks for closing the silent-typo footgun.