Allow returning from global search with the escape key

cmd+k jumps to the search tab but left users with no way back. Pressing
escape in the search field now returns to the tab they came from (never
into search itself).

Escape is handled at the search field's pressesBegan, alongside the
existing arrow-key navigation, since the focused UIKit field swallows
SwiftUI keyboard shortcuts.

ElementX/Sources/Application/Navigation/NavigationTabCoordinator.swift

@@ -100,7 +100,16 @@ import SwiftUI
     }
     
     /// The currently selected tab's tag.
-    var selectedTab: Tag?
+    var selectedTab: Tag? {
+        didSet {
+            if oldValue != selectedTab {
+                previousTab = oldValue
+            }
+        }
+    }
+    
+    /// The tab that was selected before the current one, used to return to it (e.g. cancelling search).
+    private(set) var previousTab: Tag?
     
     // MARK: Sheets