feat(BookmarkRow): adopt Dynamic Type, move cache stat off render path, safer delete

Apply WWDC26 design-brief fixes to the bookmark list reading surface:

- Dynamic Type: replace fixed .system(size:) points with text styles
  (title .headline, domain/date .footnote, summary .subheadline, tags
  .caption, podcast glyph .title3). Rows now scale and wrap with the
  user's text-size setting instead of staying fixed.
- Perf: podcastCached was a FileManager.fileExists stat evaluated inside
  body — i.e. per row, per scroll frame. Move it to @State populated by a
  .task(id:) that stats off the main actor, once per appearance.
- Forgiveness: destructive trailing swipe is now allowsFullSwipe: false so
  a single fling can't permanently delete; requires tapping the button.

Adds docs/design-wwdc26.md (consolidated brief + codebase audit).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Marks/Views/BookmarkListRow.swift

@@ -23,7 +23,9 @@ struct BookmarkListRow: View {
         .onTapGesture { onOpen() }
         .listRowInsets(EdgeInsets(top: 0, leading: 16, bottom: 0, trailing: 16))
         .listRowSeparator(.visible)
-        .swipeActions(edge: .trailing, allowsFullSwipe: true) {
+        // Delete is destructive and not undoable — require an explicit tap on the
+        // revealed button rather than letting a single full swipe delete instantly.
+        .swipeActions(edge: .trailing, allowsFullSwipe: false) {
             Button(role: .destructive) {
                 Task { await viewModel.delete(bookmark) }
             } label: {