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/GroupsTab.tsx

@@ -239,7 +239,7 @@ export default function GroupsTab({ highlightId, onHighlightConsumed }: { highli
                   onChange={(e) => setNewName(e.target.value)}
                 />
                 {duplicateGroupName && (
-                  <span style={{ color: 'var(--error)', fontSize: 12 }}>
+                  <span className={styles.errorText}>
                     Group name already exists
                   </span>
                 )}

ui/src/pages/Admin/RolesTab.tsx

@@ -130,7 +130,7 @@ export default function RolesTab({ highlightId, onHighlightConsumed }: { highlig
                   onChange={(e) => setNewName(e.target.value)}
                 />
                 {duplicateRoleName && (
-                  <span style={{ color: 'var(--error)', fontSize: 12 }}>
+                  <span className={styles.errorText}>
                     Role name already exists
                   </span>
                 )}

ui/src/pages/Admin/UserManagement.module.css

@@ -151,3 +151,8 @@
 .resetInput {
   width: 200px;
 }
+
+.errorText {
+  color: var(--error);
+  font-size: 12px;
+}

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>
                 )}