[iamdarkle]

Part of #67.

Adds an "Import accounts" flow in Settings → Storage and on the first-run screen. It reads Thunderbird for Android / K-9 Mail .k9s exports, Thunderbird profile folders, Tools › Export archives or prefs.js, and still restores Flectar backups.

• Every account is reviewed before anything is written
• Settings Flectar can't reproduce (POP3, unencrypted servers, a separate SMTP login, …) are explained instead of changed
• Gmail and Outlook accounts move to Google and Microsoft sign-in
• Sending addresses, signatures and account colors carry over
• No passwords are imported, and existing accounts are never modified

Also fixes the account editor dropping manually added sender addresses and the display name when a password is re-entered.

Also updates rustls-webpki and webpki-roots for RUSTSEC-2026-0381, -0382 and -0383, which fail the dependency audit on main.

Validation: cargo test (core and app, including text_fit), clippy -D warnings, translation check.