Client: Mermaid diagrams, drawn by the server

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Backend/Main.py

@@ -29,6 +29,7 @@ from Auth import (
 from BookReader import ReadBookForFile
 from Blame import LoadBlame, RenameUserInBlame, UpdateBlameOnSave
 from Core import NormalizeRelativePath, ResolvePath
+from Diagrams import DiagramError, DiagramsUnavailable, RenderDiagram
 from Embeds import BuildOgTags
 from Editor import WriteTextFile
 from Explorer import (
@@ -833,6 +834,25 @@ def PutSettingsEndpoint(NewSettings: dict, CurrentUser: dict = Depends(GetCurren
     return SaveSettings(CurrentUser["Username"], NewSettings)
 
 
+class DiagramRequest(BaseModel):
+    Source: str
+    Palette: dict[str, str] = {}
+    FontSize: int = 14
+
+
+# A ```mermaid block as a PNG, for the desktop client (the web view draws its
+# own). Palette is the client's theme by role, so it matches the client.
+@Application.post("/api/diagram")
+async def PostDiagramEndpoint(Body: DiagramRequest, CurrentUser: dict = Depends(GetCurrentUserOrGuest)) -> Response:
+    try:
+        Png = await RenderDiagram(Body.Source, Body.Palette, Body.FontSize)
+    except DiagramError as Error:
+        raise HTTPException(status_code=422, detail=str(Error))
+    except DiagramsUnavailable as Error:
+        raise HTTPException(status_code=503, detail=str(Error))
+    return Response(content=Png, media_type="image/png")
+
+
 @Application.get("/api/themes")
 def GetThemesEndpoint(CurrentUser: dict = Depends(GetCurrentUser)) -> list[str]:
     return ListThemeNames()