feat(#1532): default diagram theme to 'auto'; sans schema labels

Make 'auto' (adaptive light/dark SVG) the default diagram_theme, so rendered
diagrams adapt to the viewer's appearance out of the box. Also set the schema
cluster label to Helvetica — it previously fell back to Graphviz's Times default
while the node labels were already sans.

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -1691,6 +1691,7 @@ def _master_of(part_stripped):
                     style="rounded,dashed",
                     color=sc_color,
                     fontcolor=sc_fontcolor,
+                    fontname="Helvetica",  # schema label in a sans font, not Graphviz's Times default
                 )
                 node_by_name = {n.get_name().strip('"'): n for n in nodes}
                 # masters in this schema that have at least one part present

src/datajoint/settings.py

@@ -247,11 +247,11 @@ class DisplaySettings(BaseSettings):
         description="Default diagram layout direction: 'TB' (top-to-bottom) or 'LR' (left-to-right)",
     )
     diagram_theme: Literal["light", "dark", "auto"] = Field(
-        default="light",
+        default="auto",
         validation_alias="DJ_DIAGRAM_THEME",
         description=(
-            "Default diagram color theme: 'light', 'dark' (dark background with adjusted palette), "
-            "or 'auto' (single SVG that adapts to the viewer's light/dark mode)"
+            "Default diagram color theme: 'auto' (single SVG that adapts to the viewer's light/dark "
+            "mode; default), 'light', or 'dark' (dark background with adjusted palette)"
         ),
     )