Fix plus decoding in URL search hashes

[puneetdixit200]

Summary

• add a hash-fragment decoder that preserves literal + characters
• use it for URL search hash parsing in both Search and SearchScope
• cover scoped C++ URL searches and encoded literal plus queries with a focused Node test

Fixes #2463.

Tests

• node --test test/assets/search_hash_test.js
• node --check assets/javascripts/lib/util.js && node --check assets/javascripts/views/search/search.js && node --check assets/javascripts/views/search/search_scope.js && node --check test/assets/search_hash_test.js
• git diff --check

Not run locally: bundle exec rake because this checkout requires Ruby 4.0.5 and Bundler 4.0.12, while this machine only has system Ruby 2.6.10 and no matching Bundler.