gofmt: bring internal/ and cmd/ back to gofmt -l clean

Mechanical `gofmt -w ./internal ./cmd`. Mostly struct-field realignment that
had drifted, plus a few trailing-newline fixes. No behaviour change — gofmt is
semantics-preserving, and build/vet/test are green either side.

Split out from the code-review fixes that follow so those stay reviewable
instead of hiding inside a wall of realignment.

internal/plugin/adventure_arena_render.go

@@ -126,12 +126,12 @@ func renderArenaStatus(run *ArenaRun) string {
 // ── Leaderboard ─────────────────────────────────────────────────────────────
 
 type ArenaLeaderboardEntry struct {
-	DisplayName     string
-	TotalEarnings   int64
-	HighestTier     int
+	DisplayName      string
+	TotalEarnings    int64
+	HighestTier      int
 	Tier5Completions int
-	TotalRuns       int
-	TotalDeaths     int
+	TotalRuns        int
+	TotalDeaths      int
 }
 
 func renderArenaLeaderboard(season string, entries []ArenaLeaderboardEntry) string {