Create a file transfer util using Flutter for waypoint mission file copy

[spwoodcock]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Related Issue

Fixes #615

Describe this PR

• See plan.md in this PR for full context.
• This is a prototype / demo to test out the functionality proposed.
• The current mobile app isn't working well for people - I wonder if this improves things.
• The context for Flutter is explained in the issue: would be great to have Android / iOS working for this.
• I haven't used Flutter much, hence just letting an LLM just give this a crack first...

Test APK here:
dronetm-transfer.zip

AI Tool Usage

• No AI tools were used
• AI tools were used (complete below)

If AI-assisted:

• Tool(s) used: Opus 4.6 LLM
• What was generated: all of the code in this PR - it's a test / prototype
• What you reviewed and changed: Added the build script after

Review Guide

• Run build.sh to reproduce the APK.
• Install and test it!
• I don't have an RC2 controller to test this unfortunately 😢

Alternatives considered

• Fixing the dronetm-mobile app for people.
• The plan.md file here has a good analysis of what might be going wrong for them.

Checklist

• I have read the Contributing Guide
• I have read the Code of Conduct
• PR is focused and small
• Tests are included or updated
• I understand all code in this PR and can answer questions about it
• No secrets, credentials, or sensitive data are included
• Commit messages are descriptive
• Related docs and screenshots are updated

[optional] What gif best describes this PR or how it makes you feel?

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[spwoodcock]

I have read the CONTRIBUTING.md document and I hereby sign and agree with the guidelines

[spwoodcock]

@zestyping are you able to test the attached apk with the DJI RC2 controller? 🙏