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/Views/Shared/Theme.swift

@@ -165,6 +165,36 @@ struct ArticleRoute: Hashable {
     let parentHeadline: String
 }
 
+// MARK: - Appearance mode
+
+enum AppearanceMode: String, CaseIterable {
+    case system, light, dark
+
+    var colorScheme: ColorScheme? {
+        switch self {
+        case .system: return nil
+        case .light:  return .light
+        case .dark:   return .dark
+        }
+    }
+
+    var label: String {
+        switch self {
+        case .system: return "System"
+        case .light:  return "Light"
+        case .dark:   return "Dark"
+        }
+    }
+
+    var icon: String {
+        switch self {
+        case .system: return "circle.lefthalf.filled"
+        case .light:  return "sun.max"
+        case .dark:   return "moon.stars"
+        }
+    }
+}
+
 // MARK: - Stable feed ordering
 
 extension StorySummary {