lib.Uri: Return 'file' scheme for local paths

The scheme property returns 'file://' for local paths instead of 'file', causing scheme_plus_authority to produce 'file://://' with a triple slash.
Also fix the test to expect 'file' instead of 'file://'.
Assisted-by: unsloth/Qwen3.6-35B-A3B-GGUF:IQ4_NL with pi.dev v0.80.2
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/lib/Uri.py

@@ -63,7 +63,7 @@ class Uri:
     def scheme(self) -> str:
         ret = self.__p.scheme
         if not ret:
-            return 'file://'
+            return 'file'
         return ret
 
     @cached_property

test/unit/python/jw/pkg/lib/Uri/test.py

@@ -19,7 +19,7 @@ assert '<hidden>' in u.safe_full_with_username  # safe version hides password
 
 # Scheme defaults to file
 u = Uri('/local/path')
-assert u.scheme == 'file://'
+assert u.scheme == 'file'
 assert u.protocol == 'file'
 
 # Pimp returns existing Uri unchanged