Add live air-quality (US EPA AQI) chip and forecast-card row

Reuses the saved weather location's lat/lon to fetch us_aqi from
Open-Meteo's air-quality API (no key), folded into the existing 2h
forecast cache (bumped v1->v2). AQI is best-effort: a failed fetch
never sinks the forecast. Shows as a header chip and a colored row
on the forecast card; both self-hide when there is no reading.

internal/web/templates/layout.html

@@ -65,6 +65,8 @@
               class="inline-flex shrink-0 items-center gap-1.5 rounded-full bg-[color:var(--card)] px-3 py-2 text-sm font-semibold shadow-pete border-2 border-[color:var(--ink)]/10 hover:bg-[color:var(--ink)]/5 transition">
         <span data-weather-chip class="tabular-nums">📍 Weather</span>
       </button>
+      <span data-aqi-chip
+            class="hidden shrink-0 items-center gap-1.5 rounded-full bg-[color:var(--card)] px-3 py-2 text-sm font-semibold shadow-pete border-2 border-[color:var(--ink)]/10 tabular-nums"></span>
       {{if .AuthEnabled}}
         {{if .User}}
         <a href="/auth/logout" data-account