feat(webapp): admin UI for feature flag overrides (#3291)

Adds a dialog to the admin orgs page for viewing and editing per-org
feature flag overrides. Flags are introspected from the catalog so the
UI stays in sync with available flags automatically. Also adds a new tab
for global flags.

Refactors featureFlags.server.ts to split catalog definition (shared)
from server-only runtime (flags(), makeSetMultipleFlags). The shared
module exports flag metadata and validation so both the UI and API
routes can use it without pulling in server dependencies.