Add local weather forecast with live-weather backgrounds

Visitors can save a postal code (international, via Zippopotam) to get the
local forecast from Open-Meteo — a header chip + a 5-day home-page card —
and the canvas background switches from the seasonal effect to live
conditions. Entirely client-side: no keys, no server logic. Geocode cached
permanently, forecast cached 2h. Celsius by default, Fahrenheit opt-in.

New canvas effects: clear (sun by day, shaded moon + stars at night),
clouds (blurred drifting sprites), snow, fog, and storm (rain + lightning).
Seasonal effects remain the no-location fallback.

internal/web/templates/index.html

@@ -11,6 +11,8 @@
   </p>
 </section>
 
+<section data-weather-card class="mb-10 empty:hidden"></section>
+
 {{if .JustPosted}}
 <section class="mb-10">
   <div class="flex items-end justify-between gap-3 mb-4">