feat(decks): iOS-26 tabViewBottomAccessory für „Neues Deck"-Pille

Ersetzt den bottomBar-„+"-Button auf iOS 26 durch eine schwebende
Liquid-Glass-Pille via `.tabViewBottomAccessory`, nur sichtbar wenn
der Decks-Tab aktiv ist. iOS 18-Geräte behalten den bestehenden
bottomBar-Button (gated via `if #unavailable(iOS 26.0)`).

`showCreate` wandert als Binding von DeckListView nach RootView,
damit das Accessory den Sheet triggern kann.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Sources/App/RootView.swift

@@ -7,6 +7,7 @@ struct RootView: View {
     @Environment(AuthClient.self) private var auth
     @State private var selectedTab: AppTab = .decks
     @State private var pendingDeepLinkSlug: String?
+    @State private var showCreateDeck = false
 
     var body: some View {
         Group {
@@ -29,7 +30,7 @@ struct RootView: View {
     @ViewBuilder
     private var mainTabs: some View {
         TabView(selection: $selectedTab) {
-            DeckListView()
+            DeckListView(showCreate: $showCreateDeck)
                 .tabItem { Label("Decks", systemImage: "rectangle.stack") }
                 .tag(AppTab.decks)
 
@@ -43,6 +44,9 @@ struct RootView: View {
             .tabItem { Label("Account", systemImage: "person.crop.circle") }
             .tag(AppTab.account)
         }
+        .decksCreateAccessory(visible: selectedTab == .decks) {
+            showCreateDeck = true
+        }
     }
 
     /// Universal-Link- und URL-Scheme-Handler:
@@ -65,3 +69,38 @@ enum AppTab: Hashable {
     case explore
     case account
 }
+
+private extension View {
+    /// iOS 26: floating „Neues Deck"-Pille via `.tabViewBottomAccessory`,
+    /// nur sichtbar wenn der Decks-Tab aktiv ist. iOS 18 fällt auf den
+    /// bestehenden `.bottomBar`-„+"-Toolbar-Button in `DeckListView` zurück.
+    @ViewBuilder
+    func decksCreateAccessory(visible: Bool, onTap: @escaping () -> Void) -> some View {
+        if #available(iOS 26.0, *) {
+            self.tabViewBottomAccessory {
+                if visible {
+                    DeckCreateAccessoryPill(action: onTap)
+                }
+            }
+        } else {
+            self
+        }
+    }
+}
+
+@available(iOS 26.0, *)
+private struct DeckCreateAccessoryPill: View {
+    let action: () -> Void
+
+    var body: some View {
+        Button(action: action) {
+            Label("Neues Deck", systemImage: "plus")
+                .font(.subheadline.weight(.semibold))
+                .padding(.horizontal, 14)
+                .padding(.vertical, 8)
+        }
+        .buttonStyle(.borderedProminent)
+        .tint(CardsTheme.primary)
+        .accessibilityLabel("Neues Deck erstellen")
+    }
+}