v1.5.4 - Archive reliability, folder structure, and cleanup

- Fixed: archive mode now uses an explicit move instead of browser.messages.archive(), which was silently removing messages on some account types (POP3, misconfigured IMAP). Addresses a reported case of emails disappearing when a rule was set to Archive.
- Fixed: archived messages are now placed in year/month subfolders matching the native archive layout, instead of being dumped into the Archives root.
- New: "Archive into" dropdown in the rule popup (single folder, yearly folders, monthly folders). Default is monthly.
- New: pre flight check that verifies an archive folder exists before processing. If none is found, the rule aborts with a clear error instead of removing messages.
- New: error handling around archive and delete operations, with notifications on failure.
- New: warning note in the rule popup when Archive mode is selected, reminding users to configure an archive folder in the account settings.
- Improved: success notifications now show the destination folder path (for example, "Archived 194 messages from /Inbox/Leidos to /Archives/2026/2026-03").
- Documentation and metadata cleaned up to describe the add-on as a Thunderbird extension.

Minor changes:
- Adding Icons to avoid defaults.