fix(words-filter): account for British spelling (@NiketGirdhar22)

[NiketGirdhar22]

Description

The word filter previously evaluated the original word-list spelling before British English substitutions were applied. With English 1k+ languages, this could make filter presets behave incorrectly because the spelling used for filtering differed from the spelling shown during the test.
This change applies the existing British English conversion while evaluating word-filter rules. It still stores the original words in the custom word list, leaving final spelling conversion to the normal word-generation flow. The conversion helper is now synchronous because it only performs local string lookups.
Regression tests cover British English being enabled or disabled, preserving original words, and applying length filters after spelling conversion.

Checks

• Confirmed this does not add quotes, a language, a theme, a layout, or a font.
• Verified the related issue is still open and has no competing open PR.
• Used a Conventional Commits title with the contributor username.
• Built shared packages.
• Passed the focused British English and word-filter tests (14 tests).
• Passed formatting, type-aware linting, type checking, and the pre-commit hook.

Closes #6097