style: improve bottom sheet keyboard behavior with dynamic height constraints

[prateek-kalwar-95]

Description

• Add dynamic height calculation based on keyboard visibility
• Limit bottom sheet to 80% screen height when keyboard is closed
• Adjust to remaining space (minus 100px top margin) when keyboard opens
• Add BouncingScrollPhysics for smoother scrolling experience
• Keep header fixed and visible at all times
• Enable comfortable scrolling through form fields while typing

Fixes #(#534)

Screenshots

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Checklist

• [] Tests have been added or updated to cover the changes
• Documentation has been updated to reflect the changes
• Code follows the established coding style guidelines
• All tests are passing

[Mastermind-sap]

Please remove this entire commented section. You should try to avoid adding irrelevant commented section in my opinion. It is not considered a good practice.