iOS: add pH calibration UI in Calibrate tab

cue-ios/CueIOS/Views/CalibrateView.swift

@@ -15,6 +15,7 @@ struct CalibrateView: View {
             resultsSection
             cellConstantSection
             chlorineCalSection
+            phCalibrationSection
         }
         .navigationTitle("Calibrate")
     }
@@ -158,6 +159,80 @@ struct CalibrateView: View {
         }
     }
 
+    // MARK: - pH calibration
+
+    private var phCalibrationSection: some View {
+        Section("pH Calibration (Q/HQ peak-shift)") {
+            if let s = state.phSlope, let o = state.phOffset {
+                Text(String(format: "slope: %.4f mV/pH   offset: %.4f mV", s, o))
+                if let peak = detectQhqPeak(state.lsvPoints) {
+                    if abs(s) > 1e-6 {
+                        let ph = (Double(peak) - o) / s
+                        Text(String(format: "Computed pH: %.2f  (peak at %.1f mV)", ph, peak))
+                    }
+                }
+            }
+
+            HStack {
+                Text("Known pH")
+                Spacer()
+                TextField("7.00", text: $state.phCalKnown)
+                    .multilineTextAlignment(.trailing)
+                    .frame(width: 80)
+                    #if os(iOS)
+                    .keyboardType(.decimalPad)
+                    #endif
+            }
+
+            Button("Add Calibration Point") {
+                guard let peak = detectQhqPeak(state.lsvPoints) else {
+                    state.status = "No Q/HQ peak found in LSV data"
+                    return
+                }
+                guard let ph = Double(state.phCalKnown) else { return }
+                state.phCalPoints.append((ph: ph, mV: Double(peak)))
+                state.status = String(format: "pH cal point: pH=%.2f peak=%.1f mV (%d pts)",
+                    ph, peak, state.phCalPoints.count)
+            }
+            .disabled(state.lsvPoints.isEmpty)
+
+            ForEach(Array(state.phCalPoints.enumerated()), id: \.offset) { i, pt in
+                Text(String(format: "%d. pH=%.2f  peak=%.1f mV", i + 1, pt.ph, pt.mV))
+                    .font(.caption)
+            }
+
+            Button("Clear Points") {
+                state.phCalPoints.removeAll()
+                state.status = "pH cal points cleared"
+            }
+            .disabled(state.phCalPoints.isEmpty)
+
+            Button("Compute & Set pH Cal") {
+                let pts = state.phCalPoints
+                guard pts.count >= 2 else {
+                    state.status = "Need at least 2 calibration points"
+                    return
+                }
+                let n = Double(pts.count)
+                let meanPh = pts.map(\.ph).reduce(0, +) / n
+                let meanV = pts.map(\.mV).reduce(0, +) / n
+                let num = pts.map { ($0.ph - meanPh) * ($0.mV - meanV) }.reduce(0, +)
+                let den = pts.map { ($0.ph - meanPh) * ($0.ph - meanPh) }.reduce(0, +)
+                guard abs(den) > 1e-12 else {
+                    state.status = "Degenerate calibration data"
+                    return
+                }
+                let slope = num / den
+                let offset = meanV - slope * meanPh
+                state.phSlope = slope
+                state.phOffset = offset
+                state.send(buildSysexSetPhCal(Float(slope), Float(offset)))
+                state.status = String(format: "pH cal set: slope=%.4f offset=%.4f", slope, offset)
+            }
+            .disabled(state.phCalPoints.count < 2)
+        }
+    }
+
     // MARK: - Calculations
 
     private func saltGrams(volumeGal: Double, ppm: Double) -> Double {