Add ProtonMail bridge, body cache, credentials, SMTP, setup wizard, and
key development notes including the UIDVALIDITY/UIDNEXT imap-proto quirk.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Covers setup (config.toml, provider settings, Gmail app passwords),
the split-pane UI, full keyboard reference, compose/reply workflow,
auto-refresh behaviour, and a config field reference.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
