feat: added a Open image- project dialog when starting invesalius

[Prat260104]

Implements #613

Summary

This PR introduces a startup dialog displayed on application launch to guide users toward common entry points such as importing DICOM data, opening an existing project, or starting a new session.

Previously, users were presented with an empty interface without clear next steps. This change provides a structured starting point and improves overall usability, particularly for first-time users.

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Motivation

The absence of guidance on startup can lead to confusion and unnecessary friction. Introducing a startup dialog aligns InVesalius with standard UX patterns used in professional applications, making the initial workflow more intuitive and accessible.

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Implementation Details

• Added a StartupDialog using standard wxPython components to ensure full UI consistency.
• Integrated into the application startup flow after splash screen handling.
• Displayed only when appropriate:
  • Normal startup without active session or recovery
  • After canceling recovery flows
• Skipped when:
  • Crash recovery is accepted
  • A valid session is restored

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Design Considerations

• Uses native wx widgets (wx.Dialog, wx.BoxSizer, etc.)
• No custom styling; follows existing UI patterns and system theme
• Layout and behavior consistent with current dialogs
• Strings prepared for internationalization

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Testing

• Verified dialog visibility across startup scenarios
• Confirmed correct behavior of all entry actions
• Ensured no regression in crash recovery or session restore flows
• Tested on supported platforms for layout consistency

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Impact

• Provides clear entry points on startup
• Reduces initial user confusion and improves onboarding
• Maintains full UI consistency with existing components
• No breaking changes or additional dependencies

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Files Modified

• invesalius/gui/dialogs.py – Added StartupDialog
• app.py – Integrated dialog into startup flow

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Dialog UI

Screen.Recording.2026-04-18.at.21.39.08.mp4

[Prat260104]

Hi @paulojamorim sir,

This PR implements #613.

Could you please review it and share your feedback on the UI when you have time?

[Prat260104]

Hi @paulojamorim sir,
Could you please review this PR and give feedback related to UI whenever you have time ?