Organize columns by subject tags; add /subjects/ page

- Drive the columns collection from the `column` tag, recovering 3 columns
  authored under Notes/ that the glob missed
- Tag the 6 columns with subject tags; add a subjects collection and a
  /subjects/ listing page (newest-first), linked from the footer

css/style.css

@@ -175,6 +175,33 @@ a:hover {
   margin: 2rem 0;
 }
 
+/* subjects index */
+
+.subject-group {
+  margin-bottom: 2rem;
+}
+
+.subject-group h2 {
+  font-size: 1.1rem;
+  margin-bottom: 0.5rem;
+}
+
+.subject-group ul {
+  list-style: none;
+  padding-left: 0;
+}
+
+.subject-group li {
+  margin-bottom: 0.3rem;
+}
+
+.subject-group time {
+  font-family: var(--font-ui);
+  font-size: 0.8rem;
+  color: var(--color-muted);
+  margin-left: 0.5rem;
+}
+
 /* sidebar */
 
 .sidebar {