Friendlier landing for a soft-deleted práctica link (vs generic 404)

Surfaced investigating MARS-583 (db-mars-cc measurement, 2026-09-11): a práctica soft-deleted mid-session left a held/shared link 404ing for a student who hit it 5 days after the delete (d30f3e41-..., 3 hits 2026-06-10, deletedAt 2026-06-05). Only ONE confirmed real instance in 6+ months of full appEvents history - not urgent, but not-found-classify.ts already distinguishes soft-deleted-resource as its own reason, so a friendlier landing (e.g. 'esta práctica ya no está disponible' instead of the generic 404) is a small, well-scoped UI change on an existing classification branch, not new plumbing.