Improve multi-flowchart save and load handling

[legend-df]

Summary

• add dedicated actions for saving the current flowchart, saving under a new name, and saving all tabs while centralising the save logic
• reuse empty tabs when loading multiple flowcharts and keep the UI state in sync after successful loads or save attempts

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

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Suppressed error reporting when “Save Flowchart As…” fails

The new save‑as action intentionally forces saveFlowchart to run with showErrorOnFailure set to false. This means that if the user selects a path but the save actually fails (e.g. disk is full or the file is read‑only), the dialog closes silently and the user receives no warning that their changes were not written. Previously, the only save action surfaced failures. Hiding real save errors risks data loss; the action should either pass true here or detect cancelation separately so genuine failures can still be reported.

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