Move search to a dedicated tab in the tab bar

Replace the Notes list's searchable accessory with a Tab(role: .search)
— the separated magnifier at the trailing edge of the iOS 26 tab bar —
backed by a new SearchView over text, tags, and capture location. The
Notes list drops its search state and always shows the context shelf
plus all notes.

Claude-Session: https://claude.ai/code/session_01GKfAiKiQKLDTmbiGva4FGE

Notes/Views/App/ContentView.swift

@@ -19,6 +19,12 @@ struct ContentView: View {
                 Tab("Settings", systemImage: "gearshape") {
                     SettingsView()
                 }
+
+                // role: .search renders as the separated magnifier tab at the
+                // trailing edge of the tab bar on iOS 26.
+                Tab("Search", systemImage: "magnifyingglass", role: .search) {
+                    SearchView()
+                }
             }
         }
     }