[rashidazarang]

What

PNG export captures a picture of a graph, but there was no way to get the graph itself out of one browser and into another (or share one with a friend, or keep a versioned copy in a repo). This adds a portable JSON path, two icon buttons in the header:

• Download — serializes the active document to <graph-name>.json (pretty-printed, application/json), reusing the existing downloadBlob helper. Filenames are slugged from the graph name ("Retrieval — v2" → retrieval-v2.json).
• Import — file picker → parseDocumentJson, which reuses sanitizeDocument — the same integrity boundary persisted state already goes through — and returns null on malformed JSON or JSON that isn't a graph document, surfaced as a toast instead of a crash.

Design choice: import never clobbers

importGraph lands the imported document as a new graph in the active project and switches to it (mirroring createGraph: clean history, selection cleared). Importing can't destroy the graph you were looking at, and a bad file changes nothing.

Tests

• Round-trip through the integrity boundary (documentToJsonBlob → parseDocumentJson → normalizeDocument equality).
• Rejection: malformed JSON, valid-JSON-but-not-a-document, missing defaults.
• Filename slugging incl. unicode dashes and empty names.
• Store: import adds a graph, activates it, inherits the active project, starts with clean undo history.

npm test          # 33 passed + 7 companion (no new failures)
npm run build     # clean

Note: the two storage-failure tests in store.test.ts fail on main before this branch — that's #3, independent of this change.