Add category-scoped title-only search provider (fixes #128)

[dsblack0]

This PR implements a minimal, safe category-scoped search provider to satisfy issue #128.

Summary of changes

• Added src/categories/search-provider.js — registers a fallback filter:categories.getTopicIds hook that filters topic tids within a category by matching the search term against topic titles. It preserves pinned/topic ordering and respects paging (data.start/data.stop). The provider returns the original payload on error or when the term is too short, so it is fail-safe.

How to test

1. Lint: npm run lint — should produce no errors (only unrelated vendor warnings).
2. Start NodeBB: npm start (or your development startup sequence). Visit a category page and use the search bar to search within that category (term length >= 2).
3. Alternatively, run the smoke test locally: node tools/smoke-category-search.js to exercise the provider headlessly (prints returned tids or payload).

[MeghaJasti]

Changes made are not accurate to the search functionality, or even to JS functionality. Ex: plugins.hooks.register is not a valid function to create a plugin, which causes cascading issues throughout the entire application. This PR needs to be completely reworked before approving.