Issue for this PR

N/A

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

In v2 UI, disallows uploading the same file multiple times.

• Desktop file: same path + same content → duplicate.
• Browser file: same filename + same content → duplicate.
• Native desktop clipboard image: same content → duplicate.
• Same path/name, changed content: allowed.
• Same content, different path/name: allowed.
• Pasted text: never deduplicated.
• Duplicate detected: file isn’t added; toast is triggered.

How did you verify your code works?

• Ran the app locally and verified the updated UI manually
• Tests pass locally (bun run typecheck)

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

If you do not follow this template your PR will be automatically rejected.