docs: improve GitHub discoverability

[ussmarines]

Summary

Improve MailPin's GitHub discoverability without changing extension runtime behavior.

• make the root README English-first and user-oriented;
• preserve the existing French README as README.fr.md;
• add explicit Thunderbird/email productivity keywords to package.json;
• keep version guards aligned with the new root README language;
• emphasize the official Add-ons for Thunderbird install path, privacy, reminders, follow-up, inbox workflow, Kanban and local-first positioning.

No extension runtime, permissions, storage, network behavior or release version is changed.