* Preserve explicit sort overrides when searching
Closes#9466
Changes proposed in this pull request:
- Keep an explicit sort override when submitting the header search form, including `c.name`, `f.name` and `lastUserModified`.
- Continue using the already validated `FreshRSS_Context::$sort` value; omit the hidden field when no non-empty override was requested.
- Add header-rendering regression tests for all nine supported sorts, absent/empty overrides and invalid input.
* A bit more fixing
* Fix line mistake
* Forgotten plaintext
* Fix test
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Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>
