fix(drag-and-drop): track all droppable-mailbox directive instances independently

[cdvv7788]

The droppable-mailbox directive used a single shared instance variable that was overwritten each time a mailbox mounted. This meant only the last-rendered folder in the sidebar had working drag-and-drop, while all others silently failed.

Changes:

• Use an instances array instead of a single variable in the directive
• Find the correct instance by element reference on update
• Add unbind/unmounted hook to clean up listeners on unmount
• Store bound listener references so removeListeners can properly detach them
• Remove setInitialAttributes() from update() to prevent clearing drag state during Vue re-renders

Fixes #11930

I added some tests. If you run them against the old code you should be able to see the error. I am able to drop it in the new folders after the changes.

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[kesselb]

Thanks for your pr 👍

[Copilot]

Pull request overview

Fixes a drag-and-drop regression where only the last mounted droppable-mailbox directive instance worked, by tracking directive instances per element and cleaning up listeners on unmount.

Changes:

• Replace the single shared directive instance with an instances collection and look up the correct instance by el.
• Add unbind/unmounted cleanup to remove event listeners and prevent leaks.
• Add unit tests covering multiple instances, correct updates, and listener cleanup behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
src/directives/drag-and-drop/droppable-mailbox/index.js	Track multiple directive instances and add unbind/unmounted cleanup.
src/directives/drag-and-drop/droppable-mailbox/droppable-mailbox.js	Store bound listener references so removeListeners detaches correctly; avoid resetting drag state on update.
src/tests/unit/directives/droppable-mailbox.spec.js	Add tests for multi-instance behavior, correct updates, and listener removal.

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[kesselb]

Thanks a lot @cdvv7788 for fixing it.

Could you check the linter and have brief look at copilots suggestions? The update one seems valid. The "brittle" tests is accetable for me.

[cdvv7788]

I will check them, they are all relatively simple to fix.