feat(ui): add navigation links to Phase 9 features in header

Add navigation menu in header to make Phase 9 features discoverable:
- Library: Main bookshelf view
- Collections: Collection management interface
- Progress: Reading progress visualization across devices
- Devices: Device management and configuration

Navigation is hidden on mobile (responsive design) and visible on
larger screens (md breakpoint and above). This improves UX by
providing clear access to all major features.

templates/header.templ

@@ -4,11 +4,25 @@ templ Header(user User, currentPath string) {
     <nav class="border-b header-nav" style="border-color: var(--border); background-color: var(--bg-secondary)">
         <div class="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8">
             <div class="flex justify-between items-center h-16">
-                <!-- Left: App Title -->
-                <div class="flex items-center">
+                <!-- Left: App Title & Navigation -->
+                <div class="flex items-center space-x-6">
                     <a href="/bookshelf" class="text-xl font-bold hover:opacity-80 transition-opacity" style="color: var(--text-primary); text-decoration: none;">
                         📚 Bookmann
                     </a>
+                    <div class="hidden md:flex items-center space-x-4">
+                        <a href="/bookshelf" class="text-sm hover:opacity-80 transition-opacity" style="color: var(--text-secondary); text-decoration: none;">
+                            Library
+                        </a>
+                        <a href="/collections" class="text-sm hover:opacity-80 transition-opacity" style="color: var(--text-secondary); text-decoration: none;">
+                            Collections
+                        </a>
+                        <a href="/progress" class="text-sm hover:opacity-80 transition-opacity" style="color: var(--text-secondary); text-decoration: none;">
+                            Progress
+                        </a>
+                        <a href="/devices" class="text-sm hover:opacity-80 transition-opacity" style="color: var(--text-secondary); text-decoration: none;">
+                            Devices
+                        </a>
+                    </div>
                 </div>
 
                 <!-- Center: Search Box -->