perf: cache selectedCard to avoid CardDetailView re-renders

Add selectedCard as an equality-gated stored property on AppState,
updated in rebuildCards(). ContentView now reads store.state.selectedCard
instead of store.state.cards.first(where:) — this avoids observing the
entire cards array just to get the selected card, so CardDetailView
only re-renders when the selected card's data actually changes.

Author: rogeriochaves

Sources/KanbanCode/ContentView.swift

@@ -549,7 +549,7 @@ struct ContentView: View {
 
     @ViewBuilder
     private var inspectorContent: some View {
-        if let card = store.state.cards.first(where: { $0.id == store.state.selectedCardId }) {
+        if let card = store.state.selectedCard {
             makeCardDetailView(card: card)
         }
     }
@@ -573,7 +573,7 @@ struct ContentView: View {
     private var boardWithOverlays: some View {
         Group {
             if isExpandedDetail {
-                if let card = store.state.cards.first(where: { $0.id == store.state.selectedCardId }) {
+                if let card = store.state.selectedCard {
                     makeCardDetailView(card: card)
                 } else {
                     expandedEmptyState
@@ -645,7 +645,7 @@ struct ContentView: View {
             }
             .animation(.easeInOut(duration: 0.15), value: isDroppingFolder)
             .overlay {
-                if let card = store.state.cards.first(where: { $0.id == store.state.selectedCardId }),
+                if let card = store.state.selectedCard,
                    let sessionName = card.link.tmuxLink?.sessionName {
                     ImageDropZone(isTargeted: $isDroppingImage) { imageData in
                         NSPasteboard.general.clearContents()
@@ -1073,7 +1073,7 @@ struct ContentView: View {
                     }
                 }
 
-                if tbVis.showExpandedCardInfo, let card = store.state.cards.first(where: { $0.id == store.state.selectedCardId }) {
+                if tbVis.showExpandedCardInfo, let card = store.state.selectedCard {
                     ToolbarItemGroup(placement: .navigation) {
                         HStack {
                             Text("⠀⠀" + card.displayTitle)
@@ -1468,7 +1468,7 @@ struct ContentView: View {
             // Fullscreen: never close the card with Esc
             if isExpandedDetail { return }
             // Chat mode + working: send interrupt instead of closing
-            if let card = store.state.cards.first(where: { $0.id == store.state.selectedCardId }),
+            if let card = store.state.selectedCard,
                let session = card.link.tmuxLink?.sessionName,
                card.activityState == .activelyWorking || card.activityState == .idleWaiting {
                 if UserDefaults.standard.bool(forKey: "preferChatView") {

Sources/KanbanCodeCore/UseCases/BoardStore.swift

@@ -80,6 +80,10 @@ public final class AppState: @unchecked Sendable {
     /// Cached cards array — rebuilt by BoardStore after each dispatch.
     public internal(set) var cards: [KanbanCodeCard] = []
 
+    /// The currently selected card — independently tracked so CardDetailView
+    /// only re-renders when the selected card's data actually changes.
+    public internal(set) var selectedCard: KanbanCodeCard?
+
     /// Cards visible after project filtering — cached for independent observation.
     public internal(set) var filteredCards: [KanbanCodeCard] = []
 
@@ -101,6 +105,9 @@ public final class AppState: @unchecked Sendable {
         }
         if newCards != cards { cards = newCards }
 
+        let newSelected = selectedCardId.flatMap { id in cards.first { $0.id == id } }
+        if newSelected != selectedCard { selectedCard = newSelected }
+
         let newFiltered = cards.filter { cardMatchesProjectFilter($0) }
         if newFiltered != filteredCards { filteredCards = newFiltered }