Optional account, for people who want one #175

SpeakType works with no account today, and that stays true: dictation, models, history and settings never require signing in, and the app keeps working offline forever.

An account would be opt-in, and worth having for:

• Usage reports (Opt-in usage reports #176), so people who want to help can share how the app is doing.
• Sync between devices, once the phone keyboard exists. That's the only feature that genuinely needs an identity.
• A way to reach people who ask to be told about releases.

Design rules, or it isn't worth building:

• Nothing in the app is locked behind an account, now or later.
• Signing in is a choice made once, never a prompt on launch, and never a wall on first run.
• Signing out removes the local copy of the account, and deleting the account removes everything held for it.
• No account, no data leaves the machine. That promise stays exactly as it is for everyone who skips this.

Open questions: which sign-in (GitHub device flow is the least friction for this audience, email links are the most familiar), and where it runs. Whatever we pick should be documented, so people can see what's stored.