Search tab (#5754)

* Add a developer flag for the search tab
Introduces a globalSearchEnabled user preference (off by default) with a toggle in Developer Options, used to gate the new search experience.

* Support search role tabs in the tab coordinator
Migrates the NavigationTabCoordinator to SwiftUI's value-based Tab API and adds an isSearch option so a tab can adopt the iOS 26 .search role, which detaches it into the trailing search button in the tab bar.

* Moves room search to a new screen
Adds a new screen that filters the alternate room summary provider as you type, shows an empty state, and lists matching rooms with their matrix identifier. Supports hardware keyboard selection (arrow keys to move through results, return to open) by reaching into the system search field.

* Present search in a dedicated tab
Adds the search tab to the chats flow on iOS 26 when the flag is enabled, rendered as the detached .search role button. The Cmd+K shortcut now selects this tab, and the home screen's inline search is hidden while the tab is available.

* Remove the old global search screen
The dedicated search tab replaces the Cmd+K floating window, so the GlobalSearchScreen, its window plumbing in the WindowManager, and its presentation in the chats flow are all removed.

ElementX/Sources/Application/Windowing/WindowManagerProtocol.swift

@@ -50,19 +50,12 @@ protocol WindowManagerProtocol: AnyObject, OrientationManagerProtocol {
     var mainWindow: UIWindow! { get }
     /// Presented on top of the main window, to display e.g. user indicators.
     var overlayWindow: UIWindow! { get }
-    /// A window layered on top of the main one. Used by the global search function
-    var globalSearchWindow: UIWindow! { get }
     /// A secondary window that can be presented instead of the main/overlay window combo.
     var alternateWindow: UIWindow! { get }
     
     /// All the windows being managed
     var windows: [UIWindow] { get }
     
-    /// Makes the global search window key. Used to get automatic text field focus.
-    func showGlobalSearch()
-    
-    func hideGlobalSearch()
-    
     // MARK: - Secondary window support
     
     var secondaryWindowsEnabled: Bool { get set }