Stop paperSurface() undoing the serif navigation titles

Only the bookmarks screen had picked up the serif title. Every other
screen — Search, Sources, Podcasts, Settings, Ask, the sheets — was still
system bold sans, and the reason was paperSurface(): setting
.toolbarBackground makes SwiftUI build a fresh UINavigationBarAppearance
and discard the one PaperAppearance installed, text attributes included.
Bookmarks was the only screen not using the helper, which is why it alone
looked right.

The modifier no longer sets a toolbar background. It doesn't need one —
the bar is transparent by appearance and the screen already paints the
paper ground beneath it.

Also labels the bookmarks toolbar buttons (Settings, Add bookmark, Unread
filter, AI actions), which were bare SF Symbols announcing nothing to
VoiceOver.

Verified in both schemes: large titles on Bookmarks/Sources/Podcasts/
Search and inline titles on the Settings and Ask sheets all render serif,
on paper grounds.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JgLHztZGaEHvS3KNeGQmRM

Marks/Views/BookmarksView.swift

@@ -122,6 +122,7 @@ struct BookmarksView: View {
                     } label: {
                         Image(systemName: "sparkles")
                     }
+                    .accessibilityLabel("AI actions")
                 }
                 ToolbarItem(placement: .topBarTrailing) {
                     HStack(spacing: 16) {
@@ -138,6 +139,7 @@ struct BookmarksView: View {
                         Button { showAddBookmark = true } label: {
                             Image(systemName: "plus")
                         }
+                        .accessibilityLabel("Add bookmark")
                         Button {
                             Task { await viewModel.toggleUnreadFilter() }
                         } label: {
@@ -146,9 +148,11 @@ struct BookmarksView: View {
                                   : "line.3.horizontal.decrease.circle")
                                 .contentTransition(.symbolEffect(.replace))
                         }
+                        .accessibilityLabel("Unread filter")
                         Button { showSettings = true } label: {
                             Image(systemName: "gearshape")
                         }
+                        .accessibilityLabel("Settings")
                     }
                 }
             }