Docs: add initial diagram

Author: harunou

docs/ca-fe-reactive-framework-NextJs-FE-BE.svg

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+# Frontend and Backend Clean Reactive Architecture
+
+The frontend uses Clean Reactive Architecture and reaches the backend through
+its gateway. The backend follows the conventional Clean Architecture
+controller/use-case/presenter arrangement.
+
+![Frontend and Backend Clean Reactive Architecture](ca-fe-reactive-framework-NextJs-FE-BE.svg)
+
+<details>
+  <summary>mermaid</summary>
+
+```mermaid
+graph TD
+
+subgraph FE["Frontend"]
+  subgraph FB1["Boundary"]
+    FUI["User Interface"]
+  end
+
+  FPI["Presenter &lt; I &gt;"]
+  FCI["Controller &lt; I &gt;"]
+
+  FP["Presenter"]
+  FC["Controller"]
+  FUC["Use Case Interactor"]
+
+  subgraph FB3["Boundary"]
+    FE1["Entities"]
+  end
+
+  FGI["Gateway &lt; I &gt;"]
+end
+
+subgraph FBI["Frontend–Backend Integration"]
+  subgraph FB4["Boundary"]
+    FG["Gateway"]
+  end
+end
+
+subgraph BE["Backend"]
+  subgraph BB1["Boundary"]
+    BC["Controller"]
+    BP["Presenter"]
+    BVM["View Model &lt; DS &gt;"]
+  end
+
+  BID["Input Data &lt; DS &gt;"]
+  BIB["Input Boundary &lt; I &gt;"]
+  BOD["Output Data &lt; DS &gt;"]
+  BOB["Output Boundary &lt; I &gt;"]
+  BUC["Use Case Interactor"]
+
+  subgraph BB2["Boundary"]
+    BDAI["Data Access Interface &lt; I &gt;"]
+    BDA["Data Access"]
+    BDB["Database"]
+  end
+
+  subgraph BB3["Boundary"]
+    BE1["Entities"]
+  end
+end
+
+%% implementation relations
+FP -. implements .-> FPI
+FC -. implements .-> FCI
+FG -. implements .-> FGI
+BP -. implements .-> BOB
+BUC -. implements .-> BIB
+BDA -. implements .-> BDAI
+
+%% frontend dependency relations
+FUI -- depends --> FPI
+FUI -- depends --> FCI
+FC -- depends --> FUC
+FP -- depends --> FE1
+FUC -- depends --> FE1
+FUC -- depends --> FGI
+
+%% frontend-to-backend dependency relation
+FG -- depends --> BC
+
+%% backend dependency relations
+BC -- depends --> BIB
+BC -- depends --> BID
+BP -- depends --> BVM
+BP -- depends --> BOD
+BUC -- depends --> BID
+BUC -- depends --> BOB
+BUC -- depends --> BOD
+BUC -- depends --> BDAI
+BUC -- depends --> BE1
+BDAI -- depends --> BE1
+BDA -- depends --> BDB
+
+classDef boundary fill:none,stroke:#666,stroke-width:2px,stroke-dasharray: 5 5;
+class FB1,FB2,FB3,FB4,BB1,BB2,BB3 boundary;
+```
+
+</details>