Support HTML output storing and browser viewing for `kro generate diagram -f`

[Mujib-Ahasan]

Feature Description

Problem Statement:
kro generate diagram -f currently only prints HTML output to the console, which becomes hard to use for larger or more complex diagrams. There is no way to persist the output or view it in a browser-friendly format.

Proposed Solution:
Add optional support to store the generated diagram as an HTML file and open it in the browser (e.g. --open flag). Default behavior should remain unchanged to avoid breaking existing usage.

Additional Context:
Storing and optionally opening the output would significantly improve usability and align with common CLI visualization patterns.

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[Mujib-Ahasan]

Happy to work on this if this aligns with the project direction.