[Security] How to Mitigate Security Risk of Clipboard Access When Exporting Private Keys?

### What is this about?

A very serious security risk, the private key export copy can't choose to copy part of it, but only click the copy button to copy all of it, which leads to the clipboard data being accessed by other programs.
Hopefully the select partial copy function will be supported soon.

### Scenario

_No response_

### Design

_No response_

### Technical Details

_No response_

### Threat Modeling Framework

_No response_

### Acceptance Criteria

_No response_

### Stakeholder review needed before the work gets merged

- [X] Engineering (needed in most cases)
- [ ] Design
- [X] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [X] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

_No response_

No file, test, or entry point is named. Start by locating the private-key export copy flow and reviewing how clipboard access is handled. Define the threat model and acceptance criteria with Engineering, Product, and Security; done means users can copy only a selected portion without exposing the full key to the clipboard.

Written by the indexing model from the issue text.