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>

static/admin/js/init-alpine.js

@@ -57,6 +57,24 @@ function data() {
     }
   }
 
+  // ─────────────────────────────────────────────────────────────────
+  // Last visited page tracking (for redirect after login)
+  // ─────────────────────────────────────────────────────────────────
+  const LAST_PAGE_KEY = 'admin_last_visited_page';
+  const currentPath = window.location.pathname;
+
+  // Save current page (exclude login, logout, error pages)
+  if (currentPath.startsWith('/admin/') &&
+      !currentPath.includes('/login') &&
+      !currentPath.includes('/logout') &&
+      !currentPath.includes('/errors/')) {
+    try {
+      window.localStorage.setItem(LAST_PAGE_KEY, currentPath);
+    } catch (e) {
+      // Ignore storage errors
+    }
+  }
+
   // Map pages to their parent sections
   const pageSectionMap = {
     // Platform Administration