Indicate when a clip has already been exported

**Describe what you are trying to accomplish and why in non technical terms**
I want to avoid exporting the same clip multiple times. In my household, several people use Frigate in Home Assistant. Sometimes another person exports a clip that I’ve already exported (or vice versa), and we end up with duplicate or even triple exports of the same clip.
It would be helpful to have a way to clearly see if a clip has already been exported, so that users don’t waste time or storage space duplicating exports.

**Describe the solution you'd like**
An icon/marker on the clip thumbnail that indicates it has already been exported.

Alternatively (or additionally), a popup/confirmation dialog when trying to export a clip that was previously exported, asking the user if they really want to export it again.

**Describe alternatives you've considered**
Currently, the only way is to manually check exported clips and compare, which is inefficient.

**Additional context**
Multiple users in the same household exporting clips independently leads to duplicates.
Avoiding duplicates would save storage and reduce confusion.
A simple visual cue or confirmation step would greatly improve the user experience.

No files, tests, or entry points are named. Start by locating the clip thumbnail UI and the export handling in the TypeScript frontend, then determine how prior exports can be identified. Done means the chosen visual marker or confirmation flow clearly warns users about an already exported clip and is covered by appropriate tests.

Written by the indexing model from the issue text.