ci: watch npm deps in dependabot, drop dead uv ecosystem

The uv (Python) ecosystem was a leftover from the old Python project and
kept failing (dependency_file_not_found: "/ not found") since there is no
pyproject.toml. Point Dependabot at the real npm manifest instead, which
was not being watched at all.

Author: UltimaPhoenix

.github/dependabot.yml

@@ -1,8 +1,8 @@
 version: 2
 
 updates:
-  # Python dependencies (uv / pyproject.toml)
-  - package-ecosystem: uv
+  # Node dependencies (npm / package.json)
+  - package-ecosystem: npm
     directory: /
     schedule:
       interval: weekly