feat(i18n): localize basic media filters

[devy1540]

Description

This is a focused follow-up to #1318 that localizes three basic media-filter controls in the browser application. It continues the incremental localization approach used by #1383, #1384, and #1385 while remaining independently based on the latest nightly branch.

• Localize the label and options rendered by ReadStatusSelect.
• Localize the label, choices, and descriptions rendered by AgeRatingFilter.
• Use separate media and series description keys instead of interpolating an English entity name, allowing translations to use the appropriate grammar.
• Localize the label and catch-all option rendered by ExtensionSelect.
• Keep technical format names such as CBZ, EPUB, and PDF unchanged.
• Add component-oriented English source keys under a new mediaFilterForm namespace.
• Add functional regression coverage that verifies localized copy and actual react-hook-form value updates for each control.
• Add English source strings only so translated locale files continue to be managed through Weblate.

This PR preserves the existing filter values, form behavior, validation, and visible English copy. It does not change filter schemas, other filter controls, translated locale files, or mobile/Expo code.

LLM tooling assisted with this contribution. I reviewed the resulting changes and validated them locally before submission.

Validation

• Focused media-filter tests: 3 files and 4 tests passed.
• Browser test suite: 41 files and 251 tests passed.
• Browser and i18n type checks passed.
• Browser ESLint completed with 0 errors; the existing 17 unrelated warnings remain outside the changed files.
• Prettier checks passed for all changed files.
• en-US.json parses successfully.
• git diff --check passed.

Screenshots

Not included. The change is limited to replacing existing visible copy with locale lookups while preserving the rendered English text and behavior.

Ready?

• I read the contributing guidelines
• I searched for existing issues or pull requests that may be related to my contribution
• This PR is based into nightly and not main
• I added tests and/or documentation for my changes if applicable
• I disclosed any use of LLMs in the creation of this PR (if applicable)

Stump Contributor License Agreement

By contributing to Stump, I agree that these changes will be licensed under the applicable repository licenses.

[aaronleopold]

thank you