Move-folder modal: self-move guard is dead code and leaves moving stuck

[kesselb]

Summary

Flagged by Claude during a code review.
Needs validation.

Problem

onMove() sets this.moving = true, then guards the whole operation with this.mailbox.id !== this.destMailboxId. Two defects:

1. The comparison can never be true. Mailbox::jsonSerialize (lib/Db/Mailbox.php:176-177) exposes id as base64_encode($name), while destMailboxId is always the numeric databaseId or undefined. The intended "moving a folder into itself is a no-op" check therefore never fires.
2. The false branch resets nothing — no moving = false, no $emit('close'). Only the try/finally inside the if does that.

Consequences

• Selecting root for a folder that already sits at root issues a rename to its own name; the resulting IMAP error is only logged, so the user sees a silent no-op instead of the guard.
• Correcting the comparison to databaseId without also fixing the state handling would hang the dialog: since fix: minor adjustment in moving messages, threads and folders #13431 made the handler's promise the one NcDialogButton awaits, the primary button would stay disabled forever.

Suggested fix

Compare this.mailbox.databaseId against this.destMailboxId, and return before setting moving = true when they match (or hoist the finally cleanup to cover the early exit).

[GretaD]

sounds good to me. We should fix it.