feat: IMAP settings - live mailbox count and on-the-fly inbox folder selection

[joggerbra]

Problem

Currently two UX issues exist in the IMAP settings panel:

1. Mailbox count in history is only shown after the user enters the password and clicks Save Settings, i.e. an active server connection is required. It should be visible/updated without needing to save first.
2. Inbox folder selection can only be changed by clicking Save Settings. There is no way to switch the inbox folder on the fly.

Desired Behavior

• The inbox folder should be selectable via a dropdown in the settings UI at any time, without requiring a Save Settings action.
• The mailbox mail count (shown in the history view) should update live / on demand — independent of whether the user has just saved their credentials.

Acceptance Criteria

• A dropdown in the IMAP settings lets the user switch the inbox folder at any time (on the fly).
• The mailbox mail count in the history view reflects the current state without requiring a Save Settings click.
• Changes to the inbox folder selection are persisted correctly.