Improve flowchart save/load workflow

[legend-df]

Summary

• allow FlowchartPage to reuse existing file paths and expose helpers for direct save/load and detecting empty scenes
• update the Load Flowchart action to open one or more saved scenes into dedicated tabs, reusing a blank tab when possible and warning on failures

Testing

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

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