refactor: share the opportunity list filter wiring instead of hand-copying it per filter

What's wrong

Every new filter on the opportunity list has to be added by hand in six different files. Nothing in the type system catches a missed one, so forgetting a step doesn't fail the build, it just silently doesn't work at runtime. I hit this adding the skills filter in #936, and @arturasmckwcz asked me to backlog it.

What needs to happen

One config where a filter is defined once, so adding the next one is a single entry instead of six edits in six files.

The six sites a new filter has to touch.

• Opportunities/Filters/types.ts — add the key to OpportunityCardsFilter
• Opportunities/Filters/constants.ts — add the empty default
• Opportunities.tsx — createFilterFromOption entry
• Opportunities/Filters/helpers.ts — the filter block, the return, and the flat array
• Opportunities/Filters/FiltersContent.tsx — destructure plus one more AccordionFilter
• Opportunities/helpers.ts — serialize and deserialize

What I tried

I tried it on #936 and reverted it. OpportunityCardsFilter keys search, district, language and availability off QueryParamsKeys, but activity and skill off EntityTableName. The two consumers want different key types:

• generateNestedFilterControlItems wants keyof OpportunityCardsFilter
• createFilterFromOption wants keyof ApiOptionLists

So one config value can't satisfy both while the interface mixes the two enums. I didn't find a way around it without changing the filter type itself, which felt out of scope for PR #936

Question: should this cover Volunteers and Agents too?

Volunteers/helpers.ts and Agents/helpers.ts have the same shape of hand-written blocks. Should the config cover all three lists, or just Opportunities for now?
Refs #936

Start by comparing the six Opportunities files listed in the issue, then trace how OpportunityCardsFilter, QueryParamsKeys, EntityTableName, and ApiOptionLists flow through filter creation and serialization. Check Volunteers/helpers.ts and Agents/helpers.ts to resolve the intended scope. Done means a new Opportunities filter has one configuration entry and remains wired through rendering, filtering, serialization, and deserialization.