refactor: move inline styles to CSS modules

Extract inline fontSize/color styles from LogTab, LayoutShell,
UsersTab, GroupsTab, RolesTab, and LevelFilterBar into CSS modules.
Follows project convention of CSS modules over inline styles.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Admin/UsersTab.tsx

@@ -228,7 +228,7 @@ export default function UsersTab({ highlightId, onHighlightConsumed }: { highlig
                   />
                 </div>
                 {duplicateUsername && (
-                  <span style={{ color: 'var(--error)', fontSize: 12 }}>
+                  <span className={styles.errorText}>
                     Username already exists
                   </span>
                 )}