Calculator: size cables for ampacity and drop budget

recommendedCrossSection now satisfies two criteria and takes whichever
demands more copper: the voltage-drop budget, and an ampacity at least
as high as the fuse protecting the circuit (ABYC E-11: the OCP rating
must not exceed the conductor's ampacity). Ampacity comes from ABYC
E-11 Table 6A for AWG and ISO 13297 Table A1 for mm², both 105 °C.

Before this, a 40 A load over 0.5 m was sized 1.5 mm² / AWG 16 next to
a 50 A fuse: a cable the fuse can never protect. Long runs stay
voltage-drop driven and unchanged.

The drop budget is now stored per system (3 / 5 / 10 %) and editable in
the system editor, with a global default in Settings that the
standalone calculator uses and new systems inherit.

Cross-sections are derived, never entered by the user, so LoadCableSync
re-derives them when a system opens and when its budget changes.
Library loads are computed instead of being stored as a 1.0 mm²
placeholder. LoadConfigurationStatus therefore reports only what the
user can actually fix: missing length or current.

Also removes the dead cable.pro.* strings and folds three copies of the
mm²-to-AWG conversion into ElectricalCalculations.nearestAWG.

Cable/Overview/SystemOverviewView.swift

@@ -4,6 +4,7 @@ import SwiftData
 struct SystemOverviewView: View {
     @Environment(\.dismiss) private var dismiss
     @Environment(\.modelContext) private var modelContext
+
     @State private var activeStatus: LoadConfigurationStatus?
     @State private var suppressLoadNavigation = false
     @State private var showingRuntimeGoalEditor = false
@@ -390,7 +391,6 @@ struct SystemOverviewView: View {
                         .fill(Color(.tertiarySystemBackground))
                 )
             }
-            .buttonStyle(.plain)
             .alert(item: $activeStatus) { status in
                 let detail = status.detailInfo()
                 return Alert(
@@ -588,14 +588,7 @@ struct SystemOverviewView: View {
     }
 
     private var loadStatus: LoadConfigurationStatus? {
-        guard !loads.isEmpty else { return nil }
-        let incomplete = loads.filter { load in
-            load.length <= 0 || load.crossSection <= 0
-        }
-        if !incomplete.isEmpty {
-            return .missingDetails(count: incomplete.count)
-        }
-        return nil
+        LoadConfigurationStatus.evaluate(loads: loads)
     }
 
     private var totalCurrent: Double {