feat(filters): show the filter choices that have no result as unavailable

[ronjalh]

What (draft, not finished)

Peer feedback (#167, #170): show the filter choices that have no result as unavailable.

• Backend: ElementGroup.difficultyValues (schema, resolver, one extra groupBy in the structure cache, a server test).
• Frontend: a difficulty that does not exist in the chosen group, and a group with none of the chosen difficulties, are disabled in the menus, with the reason as part of the item name for a screen reader (not only dimmed, WCAG 1.4.1). Vault is left out: its difficulty is a band of D-values.
• Tests: unit tests of the two helpers, the Elements page fixture now has difficultyValues, and one e2e test (balance beam group 4 has no difficulty H).

Not done yet

• The second e2e test (a group without the chosen difficulty is disabled) was removed because its item name did not match; the helper is covered by unit tests only.
• The backend server test was written but not run (needs the database in this worktree); tsc for the frontend and the backend, and the 147 page tests, pass.
• No entry in docs/AI_USAGE.md yet, and no check with a screen reader.
• Will conflict with refactor(tests): split the ElementsPage test file by topic #191 (the Elements page test fixture moved to src/tests/elementsPage.tsx) and feat: implement pagination on Elements page #189 (pagination touches ElementsPage.tsx).

Refs: #170

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