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/UnlockView.swift

@@ -13,9 +13,10 @@ struct UnlockView: View {
     var body: some View {
         VStack(spacing: 24) {
             Spacer()
-            Image(systemName: "lock.shield.fill")
-                .font(.system(size: 64))
-                .foregroundStyle(.tint)
+            Image("AppIconImage")
+                .resizable()
+                .frame(width: 88, height: 88)
+                .clipShape(RoundedRectangle(cornerRadius: 20, style: .continuous))
             Text("KeeVault")
                 .font(.largeTitle.bold())