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/Feeds/AddFeedSheet.swift

@@ -77,7 +77,6 @@ struct AddFeedSheet: View {
             }
             .padding(.horizontal, 20)
         }
-        .preferredColorScheme(.dark)
         .presentationDetents([.medium])
         .presentationDragIndicator(.visible)
     }

Jarvis/Views/Feeds/FeedManagerView.swift

@@ -94,7 +94,6 @@ struct FeedManagerView: View {
                 feedList
             }
         }
-        .preferredColorScheme(.dark)
         .presentationDragIndicator(.visible)
         .sheet(isPresented: $showAdd) {
             AddFeedSheet { url, name in