docs(readme): add system architecture Mermaid diagram (Closes #1368)

[camesenin]

Description

Resolves issue #1368 by embedding an interactive Mermaid system architecture diagram directly into the main README.md under the ## 🔧 Architecture section.

Diagram Details:

• Clients Layer: Represents user interactions via Freighter wallet, campaign operators, and partner embedded widgets (iframe/script).
• Frontend Layer: Maps the React + Vite application responsibilities.
• Backend API Layer: Illustrates Express REST endpoints (/api/v1), metadata storage, and OpenTelemetry instrumentation.
• Blockchain / Soroban Layer: Details RPC node interactions with both the Rewards Contract (CB6KYDQ7...) and Campaign Contract (CDDVJVHP...).

Closes #1368

Type of change

• Documentation enhancement

[camesenin]

Hi @Gbangbolaoluwagbemiga @joelpeace48-cell, this PR is ready for review with all tests passing. Could you please assign me to this issue on Drips Wave (@camesenin) so that it is properly tracked for Wave 9 before merging? Thank you!