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.

OSGKeyboard/OSGKeyboardApp.swift

@@ -8,6 +8,14 @@ import OSGKeyboardShared
 struct OSGKeyboardApp: App {
     @UIApplicationDelegateAdaptor(AppURLHandler.self) private var appURLHandler
 
+    /// App-local light / dark preference (Settings ▸ Preferences ▸ Appearance).
+    @AppStorage(AppearancePreference.storageKey)
+    private var appearanceRaw = AppearancePreference.system.rawValue
+
+    private var appearance: AppearancePreference {
+        AppearancePreference.fromStored(appearanceRaw)
+    }
+
     init() {
         MaterialIconsFont.registerIfNeeded()
         if AppGroup.isAvailable {
@@ -21,10 +29,12 @@ struct OSGKeyboardApp: App {
                 ThemedRoot {
                     MainAppRoot()
                 }
+                .preferredColorScheme(appearance.colorScheme)
             } else {
                 ThemedRoot {
                     AppGroupErrorView()
                 }
+                .preferredColorScheme(appearance.colorScheme)
             }
         }
     }

OSGKeyboard/Views/MainAppRoot.swift

@@ -80,6 +80,11 @@ struct MainAppRoot: View {
         switch url.host {
         case "startflow":
             flowManager.startSession(coldStart: true)
+        #if DEBUG
+        case "seed-demo":
+            DemoDataSeeder.seedRichPlaceholderData()
+            config.reloadFromPersistedStorage()
+        #endif
         default:
             break
         }

OSGKeyboard/Views/MainSplitView.swift

@@ -54,13 +54,19 @@ struct MainSplitView: View {
 
     private var brandHeader: some View {
         HStack {
-            Image("osglogo")
+            // Match macOS sidebar: template wide mark tinted with accent so
+            // light / dark both stay readable (PNG `osglogo` can fail to
+            // show under NavigationSplitView chrome on some iPad sizes).
+            Image("OSGLogoWide")
+                .renderingMode(.template)
                 .resizable()
                 .scaledToFit()
                 .frame(height: 28)
+                .foregroundStyle(palette.accent)
                 .accessibilityLabel("OSGKeyboard")
-            Spacer()
+            Spacer(minLength: 0)
         }
+        .frame(maxWidth: .infinity, minHeight: 28, alignment: .leading)
         .padding(.leading, WideLayoutMetrics.sidebarContentInset)
         .padding(.trailing, WideLayoutMetrics.sidebarInset)
         .padding(.top, Spacing.lg)

OSGKeyboard/Views/SettingsView.swift

@@ -156,6 +156,10 @@ struct SettingsView: View {
 
                 Divider().background(palette.divider)
 
+                AppearancePickerRow()
+
+                Divider().background(palette.divider)
+
                 LocalePickerRow(
                     locales: effectiveLocales,
                     selection: Binding(
@@ -495,6 +499,27 @@ private struct AppLanguagePickerRow: View {
     }
 }
 
+// MARK: - Appearance picker row
+
+private struct AppearancePickerRow: View {
+    @AppStorage(AppearancePreference.storageKey)
+    private var appearanceRaw = AppearancePreference.system.rawValue
+
+    private var options: [(id: String, label: String)] {
+        AppearancePreference.allCases.map { preference in
+            (preference.rawValue, AppL10n.string(preference.labelKey))
+        }
+    }
+
+    var body: some View {
+        PickerRow(
+            title: AppL10n.string("settings.appearance.title"),
+            options: options,
+            selection: $appearanceRaw
+        )
+    }
+}
+
 // MARK: - Flow inactivity picker row
 
 private struct FlowInactivityPickerRow: View {

OSGKeyboard/en.lproj/Localizable.strings

@@ -91,6 +91,10 @@
 "settings.appLanguage.auto" = "Auto";
 "settings.appLanguage.english" = "English";
 "settings.appLanguage.chinese" = "Chinese";
+"settings.appearance.title" = "Appearance";
+"settings.appearance.system" = "System";
+"settings.appearance.light" = "Light";
+"settings.appearance.dark" = "Dark";
 "settings.reset.title" = "Reset all settings?";
 "settings.reset.message" = "API key, model, base URL, voice history, and cloud consent will be cleared.";
 "settings.reset.confirm" = "Reset all settings";

OSGKeyboard/zh-Hans.lproj/Localizable.strings

@@ -91,6 +91,10 @@
 "settings.appLanguage.auto" = "自动";
 "settings.appLanguage.english" = "英文";
 "settings.appLanguage.chinese" = "中文";
+"settings.appearance.title" = "外观";
+"settings.appearance.system" = "跟随系统";
+"settings.appearance.light" = "浅色";
+"settings.appearance.dark" = "深色";
 "settings.reset.title" = "重置所有设置?";
 "settings.reset.message" = "将清空 API Key、模型、接口地址、语音历史及云端确认状态。";
 "settings.reset.confirm" = "重置所有设置";