feat: drag-to-reorder assistants in the sidebar

Sources/UI/AssistantDragReorder.swift

@@ -0,0 +1,61 @@
+import SwiftUI
+import UniformTypeIdentifiers
+
+/// Drag/drop reorder for assistant tiles. Uses the legacy `.onDrag` /
+/// `.onDrop` API with the standard `public.text` UTI and an NSString
+/// payload — the modern Transferable + custom-UTType path silently
+/// dropped on macOS, and so did `.ownProcess` NSItemProvider visibility
+/// + bespoke UTI. (List ancestor + Button wrapper + visibility +
+/// custom UTI declaration each independently kill the drop.)
+struct AssistantDragReorderModifier: ViewModifier {
+    let isEnabled: Bool
+    let assistantID: String
+    let onReorder: (String) -> Void
+
+    @State private var isTargeted = false
+
+    func body(content: Content) -> some View {
+        if isEnabled {
+            content
+                .overlay {
+                    if isTargeted {
+                        RoundedRectangle(cornerRadius: JinRadius.medium, style: .continuous)
+                            .stroke(Color.accentColor, lineWidth: 2)
+                    }
+                }
+                .onDrag {
+                    NSItemProvider(object: assistantID as NSString)
+                }
+                .onDrop(of: [.text], isTargeted: $isTargeted) { providers in
+                    guard let provider = providers.first else { return false }
+                    let target = assistantID
+                    let handler = onReorder
+                    provider.loadObject(ofClass: NSString.self) { object, _ in
+                        guard let nsString = object as? NSString else { return }
+                        let sourceID = nsString as String
+                        guard !sourceID.isEmpty, sourceID != target else { return }
+                        DispatchQueue.main.async {
+                            handler(sourceID)
+                        }
+                    }
+                    return true
+                }
+        } else {
+            content
+        }
+    }
+}
+
+extension View {
+    func assistantDragReorder(
+        isEnabled: Bool,
+        assistantID: String,
+        onReorder: @escaping (String) -> Void
+    ) -> some View {
+        modifier(AssistantDragReorderModifier(
+            isEnabled: isEnabled,
+            assistantID: assistantID,
+            onReorder: onReorder
+        ))
+    }
+}

Sources/UI/ContentView+AssistantManagement.swift

@@ -67,6 +67,35 @@ extension ContentView {
         assistantPendingDeletion = nil
     }
 
+    /// Reorders `sourceID` so it skips past `targetID` in the direction it was
+    /// dragged: forward drags land the source immediately *after* the target,
+    /// backward drags land it immediately *before*. Returns `true` if the
+    /// order changed and was persisted.
+    @discardableResult
+    func reorderAssistant(sourceID: String, onto targetID: String) -> Bool {
+        guard sourceID != targetID else { return false }
+        var ordered = assistants
+        guard let sourceIdx = ordered.firstIndex(where: { $0.id == sourceID }),
+              let targetIdx = ordered.firstIndex(where: { $0.id == targetID }) else {
+            return false
+        }
+
+        let item = ordered.remove(at: sourceIdx)
+        ordered.insert(item, at: targetIdx)
+
+        let now = Date()
+        var changed = false
+        for (newOrder, assistant) in ordered.enumerated() where assistant.sortOrder != newOrder {
+            assistant.sortOrder = newOrder
+            assistant.updatedAt = now
+            changed = true
+        }
+        guard changed else { return false }
+
+        try? modelContext.save()
+        return true
+    }
+
     // MARK: - Sidebar Layout Helpers
 
     var displayedAssistants: [AssistantEntity] {