Add JSON export and import to toolbar

[tmcfarlane]

Summary

• Add "Import from JSON" option at the top of the export dropdown — opens a native file picker for .json files
• Add "Export as JSON" option — downloads the current flowchart as flowchart.json with the full { nodes, edges } structure
• Graceful error handling for invalid imports with inline error messages (invalid JSON, missing nodes/edges arrays, malformed node data)
• Import saves history before replacing the flowchart, so users can undo

Test plan

• TypeScript compiles with no errors
• All 20 existing tests pass
• Production build succeeds
• Export a flowchart as JSON, verify the downloaded file contains valid nodes/edges
• Import the exported JSON, verify the flowchart renders correctly
• Import an invalid file (e.g., a text file renamed to .json), verify a friendly error appears
• Import a JSON missing the "nodes" key, verify error message
• After importing, verify undo restores the previous flowchart

🤖 Generated with Claude Code

[vercel]

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Project	Deployment	Actions	Updated (UTC)
flowchart	Ready	Preview, Comment	Apr 13, 2026 2:42am