fix: preserve selected filter values state with reactive Set ref

[orzyyyy]

Close #3559

• Prevent resetting existedFilterValue on re-render by wrapping in ref.
• Maintain filter selection history using Vue's reactivity system.

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[webfiltered]

Once added, filters can never be removed from the existing filters set (so add, remove, add again will fail silently).

Would be good to rename it to existingFilters.

[orzyyyy]

However, when I click the remove icon, the filter is properly removed. My operation steps were: add, remove, then add again. I'm not sure if this kind of error handling is required in this scenario.

[webfiltered]

Edit: Just tested - it doesn't actually resolve the issue. Can add a filter for e.g. Comfy Core twice.