Show DTE badge in the strategy page title strip

Adds a blue "23 DTE" badge next to the strategy name in the header (or
"14 / 42 DTE" when active legs span multiple expiries — calendar/diagonal).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/strategy.html

@@ -65,6 +65,7 @@
               <h2 class="page-title">Strategy P/L Analyzer</h2>
               <div class="text-secondary mt-1" x-show="legs.length > 0" x-cloak>
                 <span class="badge bg-purple-lt fs-6 me-2" x-text="strategyName"></span>
+                <span class="badge bg-blue-lt fs-6 me-2" x-show="dteSummary" x-text="dteSummary"></span>
                 <span x-show="symbols.length <= 1" x-text="symbol"></span><span x-show="symbols.length <= 1"> · </span>
                 <span x-text="legs.length + ' leg' + (legs.length===1?'':'s')"></span> ·
                 Spot <strong class="mono" x-text="spot > 0 ? '$'+spot.toFixed(2) : '—'"></strong>
@@ -513,6 +514,14 @@
           if (d < 1) return '<1d';
           return Math.round(d) + 'd';
         },
+        get dteSummary() {
+          const uniq = [...new Set(this.activeLegs.map(l => l.expiry))];
+          if (uniq.length === 0) return '';
+          const days = uniq.map(e => Math.max(0, Math.round(legDTE({expiry:e}))));
+          if (days.length === 1) return days[0] + ' DTE';
+          days.sort((a,b) => a - b);
+          return days.join(' / ') + ' DTE';
+        },
 
         get legsView() {
           return this.legs.map(l => {