feat: remember last visited page after login

Users are now redirected to their last visited page after logging in,
instead of always going to the dashboard.

Implementation:
- Track current page in localStorage on every page load
- Exclude login, logout, onboarding, and error pages from tracking
- On login success, redirect to last visited page if valid
- Clear last visited page on logout

Admin:
- static/admin/js/init-alpine.js: Save page to admin_last_visited_page
- static/admin/js/login.js: Redirect to last page after login
- app/templates/admin/partials/header.html: Clear on logout

Vendor:
- static/vendor/js/init-alpine.js: Save page to vendor_last_visited_page
- static/vendor/js/login.js: Redirect to last page (validates vendor code)

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

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

app/templates/admin/partials/header.html

@@ -287,6 +287,7 @@ document.addEventListener('alpine:init', () => {
                     // Clear admin tokens only (not vendor or customer tokens)
                     localStorage.removeItem('admin_token');
                     localStorage.removeItem('admin_user');
+                    localStorage.removeItem('admin_last_visited_page');
                     // Note: Do NOT use localStorage.clear() - it would clear vendor/customer tokens too
                     window.location.href = '/admin/login';
                 });