Add "Import from Citavi" to Welcome Tab

[ItsYash1421]

Closes #14658

Added "Import from Citavi..." link to the Welcome Tab's Start section, making it easier for users to migrate from Citavi. When clicked, the link opens a file picker filtered to Citavi XML files (.ctv6bak, .ctv5bak) and imports the selected file(s) into a new library tab. Citation keys from Citavi are preserved (left empty) as Citavi XML exports don't contain BibTeX citation keys, allowing users to generate them later according to their preferences.

Steps to test

1. Launch JabRef
2. On the Welcome Tab, locate the "Start" section
3. Click on the new "Import from Citavi..." link (positioned after "Open library")
4. In the file picker dialog, verify that only Citavi files (.ctv6bak, .ctv5bak) are shown by default
5. Select a Citavi XML backup file (you can use test files from jablib/src/test/resources/org/jabref/logic/importer/fileformat/CitaviXmlImporterTest*.ctv6bak)
6. Verify that:
   • The file imports successfully
   • A new library tab opens with the imported entries
   • Citation keys are empty (not auto-generated)
   • A notification shows the number of imported entries
7. Test with multiple files to ensure batch import works

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• I added JUnit tests for changes (if applicable)
• I added screenshots in the PR description (if change is visible to the user)
• I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[github-actions]

Hey @ItsYash1421! 👋

Thank you for contributing to JabRef!

We have automated checks in place, based on which you will soon get feedback if any of them are failing.

After all automated checks pass, a maintainer will also review your contribution. Once that happens, you can go through their comments in the "Files changed" tab and act on them, or reply to the conversation if you have further inputs.

Please re-check our AI Usage Policy to ensure that your pull request is in line with it. It also contains links to our contribution guide in case of any other doubts related to our contribution workflow.

[koppor]

#14658 (comment)

[koppor]

Doesn't overwrite the citationkey generation settings.

(Note that we might not merge the PR, but it is still a nice excercise)

[koppor]

Use File %0 not found. See #14692

[github-actions]

Your pull request conflicts with the target branch.

Please merge with your code. For a step-by-step guide to resolve merge conflicts, see https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/addressing-merge-conflicts/resolving-a-merge-conflict-using-the-command-line.