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

Co-Authored-By: Claude <noreply@anthropic.com>

Severed/SeveredApp.swift

@@ -53,8 +53,6 @@ struct SeveredApp: App {
             RootView()
                 .environment(authorization)
                 .environment(enforcer)
-                .preferredColorScheme(.dark)
-                .tint(Theme.accent)
         }
         .modelContainer(container)
     }