feat: value source card crosslinks + gradient_map test shows input value

Add navigateToCard crosslinks for ha_entity (→ HA source), gradient_map
(→ input value source + gradient entity), and css_extract (→ color strip).
Gradient map test now charts the input interpolation factor instead of
output luminance, making the 0–1 chart meaningful.

server/src/wled_controller/api/routes/value_sources.py

@@ -397,6 +397,8 @@ async def test_value_source_ws(
                     msg["color"] = [int(r), int(g), int(b)]
                 except NotImplementedError:
                     pass
+            if hasattr(stream, "get_input_value"):
+                msg["input_value"] = round(stream.get_input_value(), 4)
             if hasattr(stream, "get_raw_value"):
                 raw = stream.get_raw_value()
                 if raw is not None: