Bug fix: persist refresh token at login so the background service worker can renew expired access tokens silently.

Major rewrite — MailStone V2 platform

• New certification workflow: pick a destination folder from
the MailStone button in the Gmail compose toolbar. The
extension auto-inserts the matching BCC address and a visual
certification header.
• Qualified eIDAS RFC 3161 timestamping + Stellar blockchain
anchoring on every certified email.
• Optional Acknowledgement of Receipt (A/R) with a tamper-
evident recipient confirmation hash.
• UI now in 5 languages: English, French, Spanish, Arabic,
Brazilian Portuguese.
• Migrated to Manifest V3 (required by Firefox).
• Security hardening: safer DOM rendering throughout, no
innerHTML interpolation in user-facing UI.

Note: this is a major version replacing v1.x. If you used the
previous extension, please log in again after the update to
re-establish your session against the new MailStone platform.