Add gasless simulation mode for educators

[collinsadi]

Educators cannot demo flows without testnet XLM and Freighter setup.

Add read-only simulation mode that walks through send/prove UI with fixture data and no chain calls.

Acceptance criteria:

• Mode clearly labeled simulation.
• No real keys derived in simulation.
• Accessible from landing page.

[octobersmvp]

@octobersmvp has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello @collinsadi! I would like to do this gasless simulation feature implementation.
Reasons for doing this:
Making an effortless onboarding walkthrough for educators is a very high-value effort. I am looking forward to doing the routing of the UI with stable fixture data.
Relevant experience:
Frontend Developer: Proficient in state management, routing conditionally, and API/blockchain mocking.
Please assign this to me!

ℹ️ Repo Maintainers: To accept this application, review their application or assign @octobersmvp to this issue.