Consolidate duplicated utilities and remove dead components

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Spec: Fowler-style refactoring review (cloud agent, 2026-08-05). Refactor only — no behavior change beyond deleting unreachable code.

What to change

Small mechanical consolidations, one PR:

1. sha256Hex ×3 — lib/export/checksum.ts, lib/offline-region/checksum.ts, and a private copy in lib/offline-region/store.ts (~38–43). One shared implementation (keep export's fail-closed policy via an option).
2. IDB promise helpers ×3 — requestToPromise/transactionDone in lib/local-capture/store.ts, lib/local-capture/media-store.ts (~49–65), lib/offline-region/pack-store.ts (~128–144). Extract lib/idb/promises.ts.
3. Byte formatters ×3, mutually incompatible — components/region-tracer.tsx formatBytes (decimal MB), components/offline-region-panel.tsx formatBytes (binary KB/MB), lib/offline-region/download-copy.ts formatRegionMegabytes. One policy in download-copy.ts; delete the locals.
4. Close-X SVG ×4+ — the m6 6 12 12M18 6 6 18 path in map-journal.tsx, daily-digest-panel.tsx, artifact-lightbox.tsx, media-lightbox.tsx, thread-chat.tsx. Extract a CloseIcon.
5. Lightbox chrome ×2 — ArtifactLightbox and MediaLightbox duplicate the Escape handling, scroll lock, and dialog frame. Extract LightboxShell (and/or a useModalLayer(onClose) hook).
6. Slugify ×2 — readTopics in lib/enrichment/system-instruction.ts (~188–200) inlines the algorithm mentionSlug (~205–212) already provides; call it. Likewise lib/artifacts/document.ts escape duplicates escapeText from lib/artifacts/sanitize.ts — export and reuse.
7. Dead code — components/offline-region-panel.tsx (OfflineRegionPanel), SheetMasthead and InstrumentStrip in components/sheet.tsx have zero product imports. Delete them (which also removes one of the byte formatters).

Out of scope: anything under app/prototype/ (throwaway by design).

Acceptance criteria

• Each listed duplicate has exactly one implementation; dead exports are removed
• mise run lint and mise run test pass