`navigator.mediaDevices` only exists in a secure context — HTTPS, or
localhost. Over plain http on a LAN address the property is undefined, so
`_startRecording` threw on its first line, the catch wrote one console line
and returned, and `_recording` stayed false: the button sat there unchanged
with nothing to read anywhere a user would look.
The unavailable cases are now named before the attempt rather than guessed
at afterwards — insecure context, unsupported browser, denied permission,
anything else — and surfaced in the chat through `_pushError`, which every
chat surface already shares. The button is deliberately still rendered when
the context is insecure: hiding it would read as "transcription is not
configured", which is the wrong diagnosis to hand someone.
Adds docs/voice.md, since "why doesn't the microphone work" is a question
the assistant will be asked and the answer is entirely outside Skald.
