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/Assets.xcassets/AccentColor.colorset/Contents.json

@@ -5,9 +5,9 @@
       "color" : {
         "color-space" : "srgb",
         "components" : {
-          "red" : "0.710",
-          "green" : "0.396",
-          "blue" : "0.114",
+          "red" : "0.420",
+          "green" : "0.271",
+          "blue" : "0.467",
           "alpha" : "1.000"
         }
       }
@@ -23,9 +23,9 @@
       "color" : {
         "color-space" : "srgb",
         "components" : {
-          "red" : "0.890",
-          "green" : "0.635",
-          "blue" : "0.357",
+          "red" : "0.651",
+          "green" : "0.498",
+          "blue" : "0.690",
           "alpha" : "1.000"
         }
       }