fix: say why the microphone is unavailable, instead of freezing the button

`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.

web/i18n/it.js

@@ -80,6 +80,10 @@ export default {
   'chat.rejected_by_user': 'Negata dall\'utente.',
   'chat.truncated':      'Risposta troncata dal limite di token (↓{tokens} tok).',
   'chat.not_connected':  'Non connesso — riconnessione in corso. Il messaggio è rimasto nella casella: premi di nuovo Invio tra un istante.',
+  'chat.mic.insecure':   'Il browser consente il microfono solo su connessione sicura. Apri Skald su http://localhost (dalla macchina stessa) oppure mettilo dietro HTTPS.',
+  'chat.mic.unsupported': 'Questo browser non supporta la registrazione vocale.',
+  'chat.mic.denied':     'Accesso al microfono negato. Consentilo per questo sito nelle impostazioni del browser e riprova.',
+  'chat.mic.failed':     'Impossibile avviare la registrazione: {error}',
 
   // ── Copilot render ─────────────────────────────────────────────────────────
   'copilot.open_in_viewer':     'Apri nel visualizzatore',