feat: switch accent to Plum, finalize vault-door app icon, group filter Clear button

- Replace Copper accent with Plum (light #6B4577 / dark #A67FB0) across the main
  app and AutoFill extension.
- Replace the padlock app icon with a vault-door design (ring, hub, locking
  handle) in Plum, plus all macOS sizes.
- Add an AppIconImage asset (separate from the AppIcon icon-set, which SwiftUI
  can't reference directly at runtime) and show it on the unlock screen instead
  of a generic SF Symbol.
- GroupFilterView: move "Clear Filter" out of the list and into a leading
  toolbar button ("Clear", disabled when no filter is active), mirroring the
  trailing "Done" button.

KeeVault/Views/GroupFilterView.swift

@@ -7,14 +7,17 @@ struct GroupFilterView: View {
 
     var body: some View {
         List {
-            if !vm.selectedGroupIds.isEmpty {
-                Button("Clear Filter", action: vm.clear)
-            }
             GroupFilterNode(group: rootGroup, vm: vm)
         }
         .scrollContentBackground(.hidden)
         .background(Color("VaultBackground"))
         .navigationTitle("Groups")
+        .toolbar {
+            ToolbarItem(placement: .cancellationAction) {
+                Button("Clear", action: vm.clear)
+                    .disabled(vm.selectedGroupIds.isEmpty)
+            }
+        }
     }
 }