feat(client): filter select_recipe by STATION_CODE with error fallback

Replace generic /api/recipes call with api_client.get_station_recipes(STATION_CODE).
Return 503 station_not_configured.html when STATION_CODE env var is unset.
Add station indicator to recipe selection page header.

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

client/templates/measure/select_recipe.html

@@ -75,6 +75,9 @@
           <p class="mt-1 text-sm text-[var(--text-secondary)]">
             {{ _('Scegli la ricetta di misura da eseguire') }}
           </p>
+          <p class="mt-1 text-sm text-steel-500 dark:text-steel-400">
+            {{ _('Stazione') }}: <span class="font-mono font-bold">{{ station_code }}</span>
+          </p>
         </div>
       </div>