Surface extension config parse errors and coerce autoWake strings

[dasomji]

Parent

Review findings on #96.

What to build

A mistake in one field of the extension config file must not silently discard the whole config. Today a parse failure is swallowed and replaced by an empty object, so writing autoWake as a string (e.g. "off", which the environment variable accepts) drops serverUrl and enables auto-wake — the opposite of intent — with no warning.

Accept the same string spellings for autoWake that the env var accepts, keep valid fields when others fail, and log a clear warning naming the invalid field.

Acceptance criteria

• autoWake accepts the same boolean-ish strings as PI_POSTBOX_AUTO_WAKE
• Invalid field → warning logged, other fields (serverUrl) retained
• docs/configuration.md updated to state accepted values
• Tests for both cases

Blocked by

• None — can start immediately

[dasomji]

Implemented and verified in #105. Closing as requested; the pull request contains the complete changes and test record.