The glass variant rendered the month title in a `.principal` ToolbarItem, which
SwiftUI drops when the state it reads (visibleMonth) changes while on screen —
it only reappeared on an unrelated rebuild (opening/closing the menu) and
vanished again on the next month change. Switched to `.navigationTitle`, which
the system updates reliably.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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