fix(frontend): don't read attachment name after delete modal closes

[tink-bot]

Attachment delete modal read attachmentToDelete.file.name unguarded in #text slot. Modal.vue keep <dialog> and slot mounted through 150ms close transition, so slot re-render after setAttachmentToDelete(null) and throw TypeError: Cannot read properties of null (reading 'file'). Sentry FRONTEND-OSS-2AY, ~212 events.

Fix: v-if="attachmentToDelete" on the <p>. Text unchanged. Modal.vue untouched. Unit test cover close-during-transition path.

How to verify

1. Open any task detail view that has at least one attachment.
2. Click the trash icon on an attachment to open the delete confirmation modal.
3. Click "Cancel" (or confirm the delete) and watch the browser console while the modal fades out.
4. Expected: the modal fades out cleanly and no error appears in the console.

Before this PR: TypeError: Cannot read properties of null (reading 'file') was thrown during the fade-out.

[github-actions]

Preview Deployment

Preview deployments for this PR are available at:

URL	Tag	Commit
https://pr-3737.preview.vikunja.dev	ghcr.io/go-vikunja/vikunja:pr-3737	latest
https://sha-c5523d4ea844009a539c37f109341c0f96c30317.preview.vikunja.dev	ghcr.io/go-vikunja/vikunja:sha-c5523d4ea844009a539c37f109341c0f96c30317	c5523d4

The preview environment will start automatically on first visit. Subsequent pushes to this PR will update the pr-3737 image — the preview picks up the new version on restart. The per-commit URLs point to a specific version and will not change.

Run locally with Docker

docker pull ghcr.io/go-vikunja/vikunja:pr-3737
docker run -p 3456:3456 ghcr.io/go-vikunja/vikunja:pr-3737

Last updated for commit c5523d4