feat(reader): redesign ArticleReaderView + global appearance toggle

Reading experience:
- Body text #CECECE (dark) / #1A1916 (light) — was near-invisible #4A4A4A
- 30pt heavy headline, 17pt body at 8.5pt line-spacing
- Skeleton pulse loading state replaces dark spinner box
- Hero image fades in on load (0.3s ease-in)
- Orange reading progress bar (2pt) below nav bar
- Max reading width 680pt for iPad legibility

Appearance:
- AppearanceMode enum (system/light/dark) in Theme.swift
- @AppStorage("appearanceMode") applied once in JarvisApp root
- Appearance picker (Picker in Menu) in reader toolbar
- Removed .preferredColorScheme(.dark) from all 12 child views

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Jarvis/JarvisApp.swift

@@ -23,6 +23,7 @@ final class AppDelegate: NSObject, UIApplicationDelegate {
 @main
 struct JarvisApp: App {
     @UIApplicationDelegateAdaptor(AppDelegate.self) private var appDelegate
+    @AppStorage("appearanceMode") private var appearanceMode: AppearanceMode = .dark
     @StateObject private var settings = ServerSettings.shared
     @StateObject private var store = StoryStore.shared
     @StateObject private var ws = WebSocketManager.shared
@@ -39,6 +40,7 @@ struct JarvisApp: App {
                     OnboardingView()
                 }
             }
+            .preferredColorScheme(appearanceMode.colorScheme)
             .environmentObject(settings)
             .environmentObject(store)
             .environmentObject(ws)