Give the tvOS media-source-not-found screen an Open Sources button

A media source destination whose source was removed rendered a bare
ContentUnavailableView, so the remote had nothing to focus.

Yattee/Localizable.xcstrings

@@ -7202,6 +7202,17 @@
         }
       }
     },
+    "navigation.sourceNotFound.openSources" : {
+      "comment" : "tvOS button on the media-source-not-found screen that opens the Sources list",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Open Sources"
+          }
+        }
+      }
+    },
     "Next" : {
 
     },

Yattee/Services/Navigation/NavigationDestination.swift

@@ -190,6 +190,14 @@ private struct MediaSourceByIDView: View {
                 Label(String(localized: "navigation.sourceNotFound"), systemImage: "externaldrive.badge.exclamationmark")
             } description: {
                 Text(String(localized: "navigation.sourceNotFound.description"))
+            } actions: {
+                #if os(tvOS)
+                // Nothing else here is focusable, and as a sidebar source this
+                // is a tab root that Menu cannot pop.
+                Button(String(localized: "navigation.sourceNotFound.openSources")) {
+                    appEnvironment?.navigationCoordinator.router.select(.sources)
+                }
+                #endif
             }
         }
     }