Support moving mail between accounts in the folder browser

[bcbetterninja]

The Move folder browser in #14 groups folders by account, but moving messages between accounts is not implemented. Track this separately for later work; #15 adds same-folder skips and responsive queued-action feedback for the existing flow.

Expected behavior

• Select a destination folder in another connected account using the existing folder browser, including for multi-selection.
• Transfer the complete message, including attachments, preserving supported metadata.
• Show per-message progress through the existing action queue without blocking navigation or resetting selection/scroll.
• Remove the source only after confirming the destination copy succeeded. Handle partial failures and retries without losing mail or creating duplicate copies.
• Skip messages already in the destination and integrate with pending-move replacement behavior.

Implementation work

• Assess supported transfer mechanisms for each source/destination account combination, including Microsoft 365 organizational and personal accounts.
• The existing Microsoft Graph message.move operation is limited to the same mailbox. Evaluate mailbox import/export support and any additional authorization/consent requirements; do not assume it supports every account type.
• Explain unsupported combinations and required authorization clearly in the folder browser.
• Add regression coverage for successful transfers, permission failures, interruption/retry, mixed-account batches, and source-deletion failure after a successful copy.

Deferred at the user’s request. This issue tracks implementation separately from the current UI improvements.