Add missing btn-secondary CSS class definition

The btn-secondary class was used 39 times across 15 templates but had
no definition in any global CSS file. The only definition existed in
error.templ's inline styles (intentionally self-contained).

Added .btn-secondary and .btn-secondary:hover to input.css using
theme-aware CSS variables (--accent) consistent with the existing
.btn-primary pattern. Rebuilt style.css via Tailwind.

web/static/input.css

@@ -148,6 +148,16 @@
   .btn-primary:hover {
     opacity: 0.8;
   }
+
+  .btn-secondary {
+    background-color: transparent;
+    color: var(--accent);
+    border: 1px solid var(--accent);
+  }
+
+  .btn-secondary:hover {
+    background-color: color-mix(in srgb, var(--accent) 10%, transparent);
+  }
 }
 
 @layer components {