Fix: Add missing .hidden CSS class to properly hide loading indicators

Root cause: JavaScript was adding/removing the 'hidden' class to toggle
visibility of loading messages and empty states, but the CSS file was
missing a general .hidden class rule. Only .message.hidden was defined.

This caused loading indicators like "Loading statistics..." and
"No finished books yet" to persist on the Reading Log page even after
data loaded successfully.

Fix: Added .hidden { display: none; } rule to style.css

Fixes #3

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/static/css/style.css

@@ -92,6 +92,11 @@ header h1 {
     border: 1px solid #f5c6cb;
 }
 
+/* General hidden class */
+.hidden {
+    display: none;
+}
+
 .message.hidden {
     display: none;
 }