Explorer: render SKOS prefLabels for facet URIs (#148 step 2)

[rdhyee]

Second step of #148 — wires the canonical `vocab_labels.parquet` artifact (PR #149) into the Interactive Explorer's facet UI.

Summary

• Loads `https://data.isamples.org/vocab_labels.parquet\` (58KB, now live behind the data-isamples-org Worker) into a URI → prefLabel `Map`.
• New `prettyLabel(uri)` helper: returns the SKOS prefLabel when known, else falls back to the last URI path segment, else the raw value. Safe against the 4 known unresolved URIs (~169 / ~6M samples — flagged as upstream design debt in Facets show vocabulary URIs instead of human-readable labels (cross-repo) #148).
• Material / Sampled Feature / Specimen Type checkboxes now render labels instead of URIs. Full URI preserved in a hover tooltip.
• Preload hint added for cache-warm parity with the other parquets.

Verification

• `curl -sI https://data.isamples.org/vocab_labels.parquet\` → 200, correct content-type, ETag set.
• Local Quarto render + Playwright smoke test on `docs/tutorials/isamples_explorer.html`: 0 JS exceptions, 0 console errors, 0 network failures.

What this doesn't do

• Doesn't fix the upstream root cause (`IdentifiedConcept.label` holds the URI in the wide parquet; `scheme_name` is NULL). That's pqg work — separate PR after we agree on whether the converter populates label from `vocab_labels` at build time, or whether consumers always JOIN.
• Doesn't yet wire labels into the other facet UIs across the site (notebooks, future detail panels). Same lookup file is reusable when we get there.

File touched

• `tutorials/isamples_explorer.qmd` — preload hint, `vocab_labels_url`, `vocabLabels` cell, `prettyLabel()` helper, three format-callback edits.