fix: prevent keyboard overlap in AI prompt on mobile

[tejaswinisa1]

PR Description

This PR fixes the Android keyboard overlap issue in the AI Prompt tab where the on-screen keyboard could cover the "System Prompt" and "User Prompt / Input" fields.

The fix:

• makes the prompt section scrollable on mobile using SingleChildScrollView
• replaces mobile-only Expanded editors with fixed-height SizedBox containers

This ensures the focused input field remains visible when the keyboard opens.

Related Issues

• Fixes On-screen keyboard overlaps input fields on Android #1381

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: This is a UI layout fix verified manually on Android by reproducing the issue and confirming proper scrolling after the fix.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux