Editing a config file as a form speaks the home page's language #9970

A JSON, YAML or TOML file opened as a form is calm and plain: the fields, the review diff and the commit step are sections and rows, not boxed cards.

Done when:

• [x] GET /:owner/:name/config/:ref/*path and both POST steps (step=review, the form coming back after an op) have no class="card inside <main>: the form, the change and the commit message are each a settings-block with an <h2>, and the diff keeps its mono config-diff lines without a box.
• [x] Add, remove and reorder of a list item, review, and save (a writer commits; anyone else proposes a pull) work as before; the existing test/config-form.test.ts and test/config-yaml-toml.test.ts stay green.
• [x] test/config-language.test.ts (new) asserts no card in <main> on the form and the review step. It fails on main today and passes with the change.
• [x] bun scripts/design-lint.ts passes; bun test is green.

Files: src/web/pages/config.ts, maybe src/web/render/config-form.ts, test/config-language.test.ts (new), docs/design/areas/<area>.md if a class is named.

Pattern: test/accounts-language.test.ts (#9948), and #9952 for a sibling page.

Risks: the form's op buttons post the whole form; keep their names and order, or the round-trip tests break.

Reviewed pull #9989 at its head: ticked GET /:owner/:name/config/:ref/*path and both POST steps (step=review, the form coming back after an op) have no class="card inside <main>: the form, the change and the commit message are each a settings-block with an <h2>, and the diff keeps its mono config-diff lines without a box.; Add, remove and reorder of a list item, review, and save (a writer commits; anyone else proposes a pull) work as before; the existing test/config-form.test.ts and test/config-yaml-toml.test.ts stay green.; test/config-language.test.ts (new) asserts no card in <main> on the form and the review step. It fails on main today and passes with the change.; bun scripts/design-lint.ts passes; bun test is green..