Fix flowchart file handling

[legend-df]

Summary

• remember the file chosen for each calculator flowchart and refresh the tab title after save/load
• expose DataFlowGraphicsScene helpers to save and load from explicit file paths that can be reused by the UI

Testing

• cmake -S . -B build (fails: missing Qt package and Catch2 download in CI environment)

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https://chatgpt.com/codex/tasks/task_e_68edb5fb39ec832d8fffe84e1c68846f

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Include QString for new save/load API

The new saveToFile/loadFromFile declarations expose QString in the public header but the header does not include <QString> or forward‑declare the type. Any TU that includes this header without already including <QString> will now fail to compile. Add the missing include or a forward declaration so the interface is self‑contained.

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