feat: add iOS support and harden receipt scanning

app/ios/ScanReceipts/Design/ZenithButtonStyle.swift

@@ -0,0 +1,96 @@
+import SwiftUI
+
+/// Buttons per `DESIGN (1).md` → Components → Buttons: "Solid #000000
+/// background with #FFFFFF text for primary actions. 1px solid #000000
+/// border with no fill for secondary. All buttons are rectangular [0px
+/// radius] with no padding-inline under 24px." And → Elevation: "On hover,
+/// elements do not lift; they shift color... a button fill turns from Black
+/// to Slate Blue-Grey" — the pressed state here plays the role "hover" plays
+/// on the web.
+/// Every Zenith button style dims to this opacity when `.disabled(true)` —
+/// centralised here so a disabled `.zenithPrimary` etc. dims automatically,
+/// the way `.borderedProminent`/`.bordered` do out of the box. Without this,
+/// each call site would need its own manual `.opacity(isEnabled ? 1 : 0.4)`,
+/// which is easy to forget (an earlier pass of this app's screens did, in
+/// fact, forget it in a couple of places before this was added centrally).
+private let zenithDisabledOpacity: Double = 0.4
+
+struct ZenithPrimaryButtonStyle: ButtonStyle {
+    var isDestructive = false
+
+    @Environment(\.isEnabled) private var isEnabled
+
+    func makeBody(configuration: Configuration) -> some View {
+        configuration.label
+            .font(.zenithBodyMd)
+            .fontWeight(.medium)
+            .foregroundStyle(.white)
+            .frame(maxWidth: .infinity, minHeight: 50)
+            .padding(.horizontal, ZenithSpacing.md)
+            .background(fill(pressed: configuration.isPressed))
+            // Sharp corners are the point of this system — a plain
+            // Rectangle fill, never `.cornerRadius`.
+            .opacity(isEnabled ? 1 : zenithDisabledOpacity)
+    }
+
+    private func fill(pressed: Bool) -> Color {
+        if isDestructive { return pressed ? Color.zenithError.opacity(0.8) : .zenithError }
+        return pressed ? .zenithSlate : .zenithBlack
+    }
+}
+
+struct ZenithSecondaryButtonStyle: ButtonStyle {
+    var isDestructive = false
+
+    @Environment(\.isEnabled) private var isEnabled
+
+    func makeBody(configuration: Configuration) -> some View {
+        configuration.label
+            .font(.zenithBodyMd)
+            .fontWeight(.medium)
+            .foregroundStyle(isDestructive ? Color.zenithError : Color.zenithBlack)
+            .frame(maxWidth: .infinity, minHeight: 50)
+            .padding(.horizontal, ZenithSpacing.md)
+            .background(Color.zenithSurface)
+            .overlay(
+                // Border THICKENS on press instead of the fill lifting —
+                // same "no lift, shift instead" rule as the primary style.
+                Rectangle()
+                    .strokeBorder(
+                        isDestructive ? Color.zenithError : Color.zenithBlack,
+                        lineWidth: configuration.isPressed ? 2 : 1
+                    )
+            )
+            .opacity(isEnabled ? 1 : zenithDisabledOpacity)
+    }
+}
+
+/// A plain-text button with no fill/border at all — for tertiary actions
+/// ("Noch kein Konto? Registrieren", "Abbrechen") that shouldn't compete
+/// visually with a screen's primary/secondary buttons.
+struct ZenithPlainButtonStyle: ButtonStyle {
+    var color: Color = .zenithMuted
+
+    @Environment(\.isEnabled) private var isEnabled
+
+    func makeBody(configuration: Configuration) -> some View {
+        configuration.label
+            .font(.zenithBodySm)
+            .foregroundStyle(configuration.isPressed ? color.opacity(0.6) : color)
+            .opacity(isEnabled ? 1 : zenithDisabledOpacity)
+    }
+}
+
+extension ButtonStyle where Self == ZenithPrimaryButtonStyle {
+    static var zenithPrimary: ZenithPrimaryButtonStyle { ZenithPrimaryButtonStyle() }
+    static var zenithPrimaryDestructive: ZenithPrimaryButtonStyle { ZenithPrimaryButtonStyle(isDestructive: true) }
+}
+
+extension ButtonStyle where Self == ZenithSecondaryButtonStyle {
+    static var zenithSecondary: ZenithSecondaryButtonStyle { ZenithSecondaryButtonStyle() }
+    static var zenithSecondaryDestructive: ZenithSecondaryButtonStyle { ZenithSecondaryButtonStyle(isDestructive: true) }
+}
+
+extension ButtonStyle where Self == ZenithPlainButtonStyle {
+    static var zenithPlain: ZenithPlainButtonStyle { ZenithPlainButtonStyle() }
+}