feat: add prompts for generating Mermaid diagrams for Flutter application architecture and use cases

docs/prompts/create-mermaid-usecase-flutter.md

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+## What the prompt does
+This prompt generates a Mermaid diagram that visualizes the use case of a Flutter application. The diagram includes the actors, use cases, and relationships between them.
+
+## Assumption
+- Flutter project is given
+- Overview mermaid diagram is given
+
+## How to use the prompt
+Using the given Flutter project code — along with the previously generated Mermaid diagram that outlines the app’s navigation and logic flow — analyze the entire system to identify **all main use cases and their sub-use cases**.
+
+From this analysis, create a **Mermaid flowchart** that shows:
+
+* All high-level use cases the app supports (e.g., Authentication, Profile Management, Dashboard Interaction, Data Fetching, Settings, etc.).
+* How each use case breaks down into sub-use cases (e.g., Authentication → Login, Signup, Logout, Token Refresh).
+* How these use cases depend on each other or trigger one another.
+* Connections between use cases as they appear in the actual code (UI actions, state changes, service calls, repository interactions, etc.).
+* Any decision points or conditions that influence which sub-use case occurs next.
+
+Format the output as a **clear Mermaid diagram** using `flowchart TD` or `flowchart LR`.
+Ensure the flow represents the **real behavior of the code**, not assumptions.
+
+Ask for any missing files if needed to build a complete and accurate use-case hierarchy.
+
+---
+
+If you want, I can also generate a version tailored for:
+✔ UML use-case diagrams
+✔ Sequence diagrams
+✔ System interaction diagrams