Auto-import and synchronize GNOME Online Accounts - #7

Summary

• automatically import mail-enabled GNOME Online Accounts and synchronize additions, removals, Mail enablement and property changes
• keep GOA authoritative for passwords/OAuth tokens; secrets remain in worker memory and are refreshed before reconnects and SMTP sends
• support separate IMAP/SMTP credentials, OAuth/XOAUTH2, SMTP auth flags, custom ports, IPv4/IPv6 and explicit implicit-TLS/STARTTLS modes
• require encrypted transports and stabilize IMAP port 143 with fail-closed STARTTLS plus a 30-second connection timeout
• preserve local sender name, signature, label, colour, emoji, enabled state and sidebar state while refreshing GOA-owned settings
• pause rather than delete an account when Mail is disabled in GNOME Settings, then restore it without losing local settings
• debounce GOA D-Bus changes and fetch snapshots off the GTK thread
• refuse to overwrite a malformed existing accounts file

Testing

• cargo check --locked
• cargo test --locked — 53 passed
• cargo clippy --all-targets --locked — passes; pre-existing style warnings remain
• combined GOA + contact-avatar build: 57 passed
• release build tested with live GOA accounts on IMAP 143 STARTTLS and IMAP 993 implicit TLS simultaneously
• air.basealt.ru:143 negotiated TLS 1.3 with certificate verification OK
• confirmed accounts.toml contains no password/token fields
• multiple independent Codex and Claude review rounds completed and findings addressed