Dedupe remaining CSS: danger colors, redundant background/cursor

- Add --danger-bg/--danger-text tokens and point .error-msg,
  .alert-error, and .btn-danger at them instead of repeating the
  same rgba/hex literals
- Drop .login-btn's cursor: pointer (already covered by button.btn)
- Drop .btn-outline's background: transparent (already covered by
  button.btn for buttons; default background for anchors)

No visual changes intended.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

styles.css

@@ -15,6 +15,9 @@
     --font-serif: 'Fraunces', Georgia, 'Times New Roman', serif;
     --font-mono: 'IBM Plex Mono', ui-monospace, Menlo, monospace;
     --font-sans: 'Karla', ui-sans-serif, system-ui, sans-serif;
+
+    --danger-bg: rgba(180, 50, 40, 0.15);
+    --danger-text: #ff9d9d;
   }
 
   * {
@@ -486,7 +489,6 @@
   .btn-outline {
     border: 1px solid var(--gold-dim);
     color: var(--gold-bright);
-    background: transparent;
   }
 
   /* .nav-cta mirrors this hover treatment */
@@ -985,9 +987,9 @@
 
   .error-msg,
   .alert-error {
-    background: rgba(180, 50, 40, 0.15);
+    background: var(--danger-bg);
     border: 1px solid rgba(220, 80, 60, 0.4);
-    color: #ff9d9d;
+    color: var(--danger-text);
     padding: 0.85rem 1.1rem;
     font-size: 0.88rem;
     border-radius: 0.125rem;
@@ -1007,7 +1009,6 @@
     width: 100%;
     justify-content: center;
     margin-top: 0.5rem;
-    cursor: pointer;
   }
 
   .back-link {
@@ -1215,9 +1216,9 @@
   }
 
   .btn-danger {
-    background: rgba(180, 50, 40, 0.15);
+    background: var(--danger-bg);
     border: 1px solid rgba(220, 80, 60, 0.5);
-    color: #ff9d9d;
+    color: var(--danger-text);
   }
 
   .btn-danger:hover {