Label the tag-pin button for VoiceOver

The + in the filter strip was an unlabeled SF Symbol, so VoiceOver
announced nothing useful for it. Also makes the control queryable, which
is how the menu was verified.

Verified by driving the real UI with a temporary XCUITest target: tapping
+ opens a menu listing the available tags, and tapping a card opens the
browser. The target was removed afterwards rather than committed — both
tests need the live linkding server to have loaded bookmarks first, which
is not something the unit suite should depend on.

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

Marks/Views/Library/LibraryKit.swift

@@ -308,6 +308,7 @@ struct LibraryTagStrip: View {
                             .foregroundStyle(Paper.ink.opacity(0.6))
                             .frame(width: 50, height: 42)
                     }
+                    .accessibilityLabel("Pin a tag")
                     Spacer(minLength: 0)
                 }
                 .padding(.horizontal, 18)