fix(ui): no-op when re-selecting the turn already on screen

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>

ui/docs/navigation.md

@@ -179,8 +179,12 @@ arrows step `viewedTurn` by ±1 (disabled at boundaries `0` and
 popover (desktop) or a fixed full-width drawer (mobile, ≤ 767.98
 px) listing every turn from `currentTurn` down to `0`. Selecting
 the current-turn row routes through `gameState.returnToCurrent()`;
-any other row calls `gameState.viewTurn(N)`. The popover reuses
-`view-menu.svelte`'s outside-click / Escape pattern.
+any other row calls `gameState.viewTurn(N)`. Selecting the row
+already on screen (`viewedTurn`) is a pure no-op — it only closes
+the popover — so re-picking the live turn (most visibly turn 0,
+where it is the only row) never re-fetches the report to redraw the
+same snapshot. The popover reuses `view-menu.svelte`'s
+outside-click / Escape pattern.
 
 `lib/header/history-banner.svelte` renders directly under the
 header whenever `gameState.historyMode === true`. It shows