feat: iOS appearance, iPad logo fix, demo seed, and landing hero polish

Add System/Light/Dark preference for iPhone and iPad, restore the iPad
sidebar brand mark, ship a DEBUG seed-demo URL for rich placeholder data,
and refresh GitHub Pages with the device composite, full-bleed pale-green
hero wash, and transparent Mac screenshot chrome.

OSGKeyboardShared/Models/AppearancePreference.swift

@@ -0,0 +1,41 @@
+// AppearancePreference.swift
+// OSGKeyboard · Shared
+//
+// In-app light / dark preference for the iOS main app (iPhone + iPad).
+// Mirrors macOS `MacAppearancePreference` but uses iOS Settings copy keys.
+// Stored in standard UserDefaults (app-local); not synced via iCloud.
+
+import SwiftUI
+
+/// How the iOS host app resolves its colour scheme.
+public enum AppearancePreference: String, CaseIterable, Identifiable, Sendable, Codable {
+    case system
+    case light
+    case dark
+
+    public var id: String { rawValue }
+
+    /// `nil` means follow the system — SwiftUI's `preferredColorScheme(nil)`.
+    public var colorScheme: ColorScheme? {
+        switch self {
+        case .system: return nil
+        case .light:  return .light
+        case .dark:   return .dark
+        }
+    }
+
+    public var labelKey: String {
+        switch self {
+        case .system: return "settings.appearance.system"
+        case .light:  return "settings.appearance.light"
+        case .dark:   return "settings.appearance.dark"
+        }
+    }
+
+    public static let storageKey = "config.appearancePreference"
+
+    public static func fromStored(_ raw: String?) -> AppearancePreference {
+        guard let raw, let value = AppearancePreference(rawValue: raw) else { return .system }
+        return value
+    }
+}