fix: broken functionality - auth errors, toast logic, role update, routing, step library

- Extract backend error detail in auth store login/register
- Fix inverted 4xx toast logic and add 429 rate limit handling
- Send account_role field to match backend schema in role update
- Use type-aware routing for Repeat Last Session button
- Add step library placeholder page and route, remove dot badge

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/Sidebar.tsx

@@ -172,7 +172,7 @@ export function Sidebar() {
             <NavItem href="/my-trees" icon={PenLine} label="Flow Editor" />
             <NavItem href="/sessions" icon={Clock} label="Sessions" badge={activeSessionCount || undefined} />
             <NavItem href="/shares" icon={FileText} label="Exports" />
-            <NavItem href="/step-library" icon={Bookmark} label="Step Library" badge="dot" />
+            <NavItem href="/step-library" icon={Bookmark} label="Step Library" />
             <NavItem href="/analytics" icon={BarChart3} label="Analytics" />
           </div>