Fix macOS native styling for search toolbar pickers and view options

The content type segmented picker rendered as a Liquid-Glass pill instead
of a native macOS segmented control. Wrapping a `.segmented` Picker in
`.opacity`/`.disabled`/`.accessibilityHidden`/`.fixedSize` (used to keep
toolbar items mounted-but-invisible for stable layout) inserts a hosting
layer that makes the toolbar fall back to the pill rendering.

- SearchView: filters button and content type picker are now always
  visible on macOS, rendered bare so `.segmented` keeps native styling.
- SearchView: don't reset the content type to .video on submit on macOS,
  so a pre-selected type (e.g. Playlists) is preserved (iOS unchanged).
- InstanceBrowseView: same pill fix, rendering the tab / content type
  pickers bare via conditional `if` instead of opacity-masking.
- ViewOptionsSheet / MediaBrowserViewOptionsSheet: use `.formStyle(.grouped)`
  for native macOS form styling in the options popover.

Yattee/Views/MediaBrowser/MediaBrowserViewOptionsSheet.swift

@@ -24,7 +24,7 @@ struct MediaBrowserViewOptionsSheet: View {
         #if os(macOS)
         // Popover content: no navigation chrome, click-outside dismisses.
         formContent
-            .padding()
+            .formStyle(.grouped)
             .frame(width: 300)
             .onAppear {
                 // Reset sort order if current selection is not available for this source type