fix(frontend): plans provider filter offers 'multiple' instead of bound plans' providers #1851

Follow-up from the review of #1843 (part of #1746, bound plans).

What: a bound plan whose groups span providers shows provider multiple on its planned-purchase row, and lists the real providers in providers (#1843). The plans column filter now matches a row on provider or any entry in providers (rowMatchesProvider, frontend/src/lib/upcoming-scope.ts). But the provider column's dropdown options are still built from the raw provider values only. As a result:

• multiple appears as an option, which means nothing to a user;
• a provider that occurs only inside a bound plan's providers (for example azure, when no other row is Azure) is not offered, so that plan can't be filtered to through the dropdown.

Expected: the options are the union of every row's provider and providers, without multiple. Selecting one shows each bound plan that spans it, which rowMatchesProvider already handles.

Where: the provider column's option builder in frontend/src/plans.ts (the column filter setup).

Impact today: latent. No API path creates bound plans yet (slice A of #1746 lands the backend), so no user sees a multiple row until bound-plan creation ships. Fix it before then.

Refs #1746, #1843.