Fix webapp Pyodide compatibility: pin matplotlib and contourpy versions

The webapp uses Shinylive which runs Python in the browser via Pyodide.
Pyodide only supports specific versions of packages that have C extensions.
Pin matplotlib to 3.8.4 and contourpy to 1.3.1 to match Pyodide's available versions.

This fixes the "Can't find a pure Python 3 wheel for 'contourpy==1.3.2'" error
when loading the webapp.

webapp/requirements.txt

@@ -1,5 +1,5 @@
 # This file was autogenerated by uv via the following command:
-#    uv pip compile --output-file=requirements.txt requirements.txt.in
+#    uv pip compile requirements.txt.in --output-file requirements.txt
 anyio==4.9.0
     # via
     #   starlette
@@ -14,8 +14,10 @@ click==8.2.1
     # via
     #   shiny
     #   uvicorn
-contourpy==1.3.2
-    # via matplotlib
+contourpy==1.3.1
+    # via
+    #   -r requirements.txt.in
+    #   matplotlib
 cycler==0.12.1
     # via matplotlib
 fonttools==4.58.5
@@ -36,7 +38,7 @@ markdown-it-py==3.0.0
     # via
     #   mdit-py-plugins
     #   shiny
-matplotlib==3.10.3
+matplotlib==3.8.4
     # via -r requirements.txt.in
 mdit-py-plugins==0.4.2
     # via shiny

webapp/requirements.txt.in

@@ -1,4 +1,5 @@
 astral==2.2
-matplotlib
+matplotlib==3.8.4
+contourpy==1.3.1
 shinyswatch
 shiny