Reset back-press callback on Loading and Empty in ChatHistoryFragment

The callback's isEnabled flag was only assigned inside the Loaded branch of
render(); a transition out of Loaded (e.g. last chat deleted externally while
in select mode) left it enabled but with no matching case in handleOnBackPressed,
silently consuming every back press and the toolbar nav arrow.

Re-derive isEnabled from the rendered state at the end of render() via a pure
helper, so every transition recomputes it and any future state defaults to off.

Author: GerardPaligot

duckchat/duckchat-impl/src/main/java/com/duckduckgo/duckchat/impl/history/ChatHistoryFragment.kt

@@ -143,16 +143,23 @@ class ChatHistoryFragment : DuckDuckGoFragment(R.layout.fragment_chat_history) {
                 if (selectMode != null) {
                     applySelectModeToolbar(selectMode.selectedChatIds.size)
                     setFireActionVisible(selectMode.selectedChatIds.isNotEmpty())
-                    onBackPressedCallback.isEnabled = true
                 } else {
                     applyDefaultToolbar()
                     // Hide fire when Recent is empty — title is "Delete N chats?" of the Recent count.
                     setFireActionVisible(state.recent.isNotEmpty())
-                    onBackPressedCallback.isEnabled = binding.searchBar.isVisible
                 }
                 renderConfirmation(state.confirmation)
             }
         }
+        // Re-derive every render so a transition out of Loaded (e.g. last chat deleted externally)
+        // can't leave us intercepting back presses with no overlay to dismiss.
+        onBackPressedCallback.isEnabled = shouldInterceptBack(state)
+    }
+
+    private fun shouldInterceptBack(state: ChatHistoryUiState): Boolean {
+        if (binding.searchBar.isVisible) return true
+        val loaded = state as? ChatHistoryUiState.Loaded ?: return false
+        return loaded.mode is ChatHistoryUiState.Mode.Selecting
     }
 
     private fun applyDefaultToolbar() {