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/layout.html

@@ -60,6 +60,11 @@
         <span class="sr-only">Toggle weather animation</span>
       </button>
       {{end}}
+      <button type="button" data-weather-loc
+              title="Set your location"
+              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>
       <div class="flex items-center gap-2 min-w-0 max-w-full">
         <button type="button" data-search-trigger title="Search (⌘K)"
                 class="flex shrink-0 items-center gap-2 rounded-full bg-[color:var(--card)] px-3 py-2 text-sm shadow-pete border-2 border-[color:var(--ink)]/10 hover:bg-[color:var(--ink)]/5 transition">
@@ -145,6 +150,7 @@
 
   <script>window.PETE_SOURCES = {{.AllSources}};</script>
   <script src="/static/js/weather.js" defer></script>
+  <script src="/static/js/weather-forecast.js" defer></script>
   <script src="/static/js/search.js" defer></script>
   <script src="/static/js/settings.js" defer></script>
 </body>