fix(schematic): correct power input connector and namespace RHL0020A footprint

Replace the 3-pos MaiXu MX126-5.0 terminal block footprint on the power
input connector with a 3-pos JST EH vertical header, matching the actual
battery harness. Namespace the RHL0020A footprint reference as
Project:RHL0020A so it resolves through the project-local Project.pretty
library rather than the previous unqualified bare name.

Constraint: RHL0020A footprint lives in the project-local library
Confidence: high
Scope-risk: narrow

firstpcb.kicad_sch

@@ -5925,7 +5925,7 @@
 				)
 			)
 		)
-		(property "Footprint" "RHL0020A"
+		(property "Footprint" "Project:RHL0020A"
 			(at 172.72 127 0)
 			(hide yes)
 			(show_name no)
@@ -7808,7 +7808,7 @@
 				(justify left)
 			)
 		)
-		(property "Footprint" "TerminalBlock:TerminalBlock_MaiXu_MX126-5.0-03P_1x03_P5.00mm"
+		(property "Footprint" "Connector_JST:JST_EH_B3B-EH-A_1x03_P2.50mm_Vertical"
 			(at 144.78 31.75 0)
 			(hide yes)
 			(show_name no)