refactor: re-skin UI with native iOS components

Replace the Opal-style custom presentation with the bare iOS design
language, keeping the backend, flows, and accessibility identifiers:

- Home: NavigationStack + List; rules now presented as list rows with
  kind icon, block summary, and trailing live status; '+' in the toolbar
- New Rule: plain list of rule types and presets; editor still pushed
  via navigationDestination
- Editor: native Form (DatePicker rows, day circles with summary in the
  section header, toggle rows with footers, stepper rows); create
  commits with a prominent 'Add Rule' button replacing Hold to Commit;
  edit uses toolbar Done plus red Disable/Delete rows
- Detail: sheet with inline title + status caption, LabeledContent
  rows; Edit Rule pushes the editor natively
- Default color scheme: drop forced dark mode and custom tint; system
  appearance and accent throughout
- Delete Theme, HoldToCommitButton, RuleCardView and custom chrome
- Use concrete Color.primary/secondary in button row labels (the
  hierarchical styles resolve against the button tint)
- Tests: 93 passing; only delta is holdToCommitButton press →
  commitRuleButton tap in 5 call sites
- Spec: add §6 mapping the original presentation to the native one

Severed/Views/Onboarding/OnboardingView.swift

@@ -5,9 +5,9 @@
 
 import SwiftUI
 
-/// Two-step onboarding: a welcome screen, then the Screen Time permission
-/// request. Onboarding only completes once authorization is approved — the
-/// app cannot block anything without it.
+/// Two-step onboarding using system styling: a welcome screen, then the
+/// Screen Time permission request. Onboarding only completes once
+/// authorization is approved — the app cannot block anything without it.
 struct OnboardingView: View {
     @Environment(ScreenTimeAuthorization.self) private var authorization
     @Environment(\.openURL) private var openURL
@@ -31,23 +31,18 @@ struct OnboardingView: View {
             Spacer()
             footer
         }
-        .padding(.horizontal, 28)
-        .padding(.bottom, 24)
-        .frame(maxWidth: .infinity, maxHeight: .infinity)
-        .foregroundStyle(.white)
-        .background(Theme.background)
+        .padding()
     }
 
     private var welcome: some View {
-        VStack(spacing: 18) {
+        VStack(spacing: 16) {
             Image(systemName: "scissors")
-                .font(.system(size: 56, weight: .semibold))
-                .foregroundStyle(Theme.accent)
+                .font(.system(size: 56))
+                .foregroundStyle(.tint)
             Text("Severed")
-                .font(.system(size: 38, weight: .bold))
+                .font(.largeTitle.bold())
             Text("Block your most distracting apps with rules that keep you honest — on a schedule, with no way out when you choose Hard Mode.")
-                .font(.system(size: 16))
-                .foregroundStyle(Theme.textSecondary)
+                .foregroundStyle(.secondary)
                 .multilineTextAlignment(.center)
         }
     }
@@ -55,10 +50,10 @@ struct OnboardingView: View {
     private var permission: some View {
         VStack(spacing: 24) {
             Image(systemName: "hourglass")
-                .font(.system(size: 48, weight: .semibold))
-                .foregroundStyle(Theme.accent)
+                .font(.system(size: 48))
+                .foregroundStyle(.tint)
             Text("Allow Screen Time Access")
-                .font(.system(size: 28, weight: .bold))
+                .font(.title.bold())
                 .multilineTextAlignment(.center)
             VStack(alignment: .leading, spacing: 14) {
                 bullet("shield.fill", "Severed uses Apple's Screen Time framework to block the apps you choose.")
@@ -68,8 +63,8 @@ struct OnboardingView: View {
             if authorization.status == .denied || authorization.lastRequestFailed {
                 VStack(spacing: 10) {
                     Text("Screen Time access was declined. Severed can't block apps without it.")
-                        .font(.system(size: 13))
-                        .foregroundStyle(Theme.destructive)
+                        .font(.footnote)
+                        .foregroundStyle(.red)
                         .multilineTextAlignment(.center)
                         .accessibilityIdentifier("permissionDeniedLabel")
                     Button("Open Settings") {
@@ -77,8 +72,6 @@ struct OnboardingView: View {
                             openURL(url)
                         }
                     }
-                    .font(.system(size: 15, weight: .semibold))
-                    .foregroundStyle(Theme.accent)
                     .accessibilityIdentifier("openSettingsButton")
                 }
             }
@@ -88,12 +81,11 @@ struct OnboardingView: View {
     private func bullet(_ systemImage: String, _ text: String) -> some View {
         HStack(alignment: .top, spacing: 12) {
             Image(systemName: systemImage)
-                .font(.system(size: 15, weight: .semibold))
-                .foregroundStyle(Theme.accent)
-                .frame(width: 22)
+                .foregroundStyle(.tint)
+                .frame(width: 24)
             Text(text)
-                .font(.system(size: 15))
-                .foregroundStyle(Theme.textSecondary)
+                .font(.subheadline)
+                .foregroundStyle(.secondary)
         }
     }
 
@@ -127,13 +119,10 @@ struct OnboardingView: View {
     ) -> some View {
         Button(action: action) {
             Text(title)
-                .font(.system(size: 17, weight: .semibold))
-                .foregroundStyle(.black)
                 .frame(maxWidth: .infinity)
-                .frame(height: 54)
-                .background(.white, in: RoundedRectangle(cornerRadius: 27))
         }
-        .buttonStyle(.plain)
+        .buttonStyle(.borderedProminent)
+        .controlSize(.large)
         .accessibilityIdentifier(identifier)
     }
 }