Apply Emil Kowalski design across all screens

Near-monochrome palette (ink tokens), black primary buttons, spring
animations, 0.5pt hairline dividers, shadow cards, squircle corners.
Replaces all blue/coloured accents. Adds ButtonStyles component.

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

Features/MainTabView.swift

@@ -40,7 +40,7 @@ struct iPhoneTabLayout: View {
             NavigationStack { SettingsView() }
                 .tabItem { Label("Settings", systemImage: "gearshape.fill") }
         }
-        .tint(AppTheme.blue)
+        .tint(AppTheme.ink)
     }
 }
 
@@ -68,6 +68,6 @@ struct iPadSidebarLayout: View {
             default: BackupView()
             }
         }
-        .tint(AppTheme.blue)
+        .tint(AppTheme.ink)
     }
 }