feat: one Missing files page under Collection: folders gone, each with its album's want, last search or download, and Search, Releases and Remove; an album held before its files went is no longer shown as held again (M504 #5626)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/src/test/java/com/fabledsword/minstrel/notifications/ui/NotificationUiTest.kt

@@ -27,7 +27,7 @@ class NotificationUiTest {
         // Library health pages the app has no screen for open the Admin landing.
         assertEquals(Admin, routeForLink("/collection/duplicates"))
         assertEquals(Admin, routeForLink("/collection/duplicates?view=resolved"))
-        assertEquals(Admin, routeForLink("/admin/missing-files"))
+        assertEquals(Admin, routeForLink("/collection/missing"))
         assertEquals(Admin, routeForLink("/admin"))
         assertNull(routeForLink("/somewhere-new"))
     }