AO3-7357 i18n SearchHelper#search_header

[not-varram]

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Issue

https://otwarchive.atlassian.net/browse/AO3-7357

Purpose

Rewrote SearchHelper#search_header to fix three i18n issues: pluralisation now uses explicit Rails one/other keys per item type, string concatenation isreplaced with full-phrase translation keys so locales can define their own word order, and each item type now has its own key tree instead of a word being inserted into a shared string.

The method signature was updated to use keyword arguments; all call sites in views were updated accordingly.

Testing Instructions

Automated coverage was added in spec/helpers/search_helper_spec.rb.

(Functional QA flow is documented in the Jira ticket.)

Credit

varram (he/him)