Add opt-in migration from nostr-article-capture userscript storage

[bryanmatthewsimonson]

If a user was running the nostr-article-capture userscript (v1.8.0 same schema), they've got publications / people / organizations / keypair registry / preferences stored in their user-script manager's GM_setValue area. The extension can't read that directly (different storage scope), but we can offer a frictionless way to export-from-userscript then import-into-extension.

Plan

1. In the userscript, add an "Export all data for X-Ray" menu command that writes a single combined JSON blob to the clipboard (or downloads a file): { publications, people, organizations, keypair_registry, preferences }.
2. In the options page, add an "Import from userscript" flow that accepts that blob and merges each section into chrome.storage.local (preserving existing extension data).

Notes

• Keypair export already has a single-section path (Keypair Registry → Import JSON). This is the broader version for everything-at-once.
• Warn clearly that the blob contains private keys; treat it the same as the existing keypair export.
• Doesn't need to be symmetric — we only need userscript → extension, not the reverse.

[bryanmatthewsimonson]

Userscript-to-extension storage migration remains valid in principle, but its priority drops during the v4.2 parity push (roadmap: #20) — the entity schema and storage shape are both changing in Phases 4 and 6, so any migration helper written now would be a migration from userscript v1 to extension v1, both of which are intermediate states. Revisit once Phase 6 (#17) lands and the extension's storage schema matches the userscript's.