Clicking the current-turn row in the header turn navigator while
already viewing it routed through returnToCurrent() →
viewTurn(currentTurn), which re-fetches the live report and flips the
view through `loading`. At turn 0 the only row is the live turn, so
the dropdown always fired a pointless backend round-trip and redraw.
Guard goToTurn() against re-selecting the on-screen turn
(turn === viewedTurn): just close the popover and stop. Leaving
history is unaffected — there the viewed turn differs from the target.
Closes#45
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>