feat: add loading spinner to species search button

Shows Bootstrap spinner via htmx-indicator during the Perenual API
call, and disables the button to prevent double-submits.

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

plants/templates/plants/species_search.html

@@ -19,7 +19,12 @@
         hx-include="#species-q"
         hx-target="#species-results"
         hx-swap="innerHTML"
-    >Search</button>
+        hx-indicator="#search-spinner"
+        hx-disabled-elt="this"
+    >
+        <span id="search-spinner" class="htmx-indicator spinner-border spinner-border-sm me-1" role="status"></span>
+        Search
+    </button>
 </div>
 
 <div id="species-results" class="list-group mb-3"></div>