Fix awkward wrapping in post metadata (#1790)

The previous shared styling applied flex layout to both `.post-meta` and
`.breadcrumbs`, causing awkward line breaks in post metadata where
separators and reading time would appear at the start of new lines.

This change keeps the shared color and font-size styling, but only
applies flex layout to `.breadcrumbs`. Post metadata (author, date,
reading time) uses the default block flow which handles inline content
with separators more naturally.

Fixes #1789

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

assets/css/common/post-single.css

@@ -17,6 +17,9 @@
 .breadcrumbs {
     color: var(--secondary);
     font-size: 14px;
+}
+
+.breadcrumbs {
     display: flex;
     flex-wrap: wrap;
     align-items: center;