feat: Implement modular plugin architecture

- Convert invcount to self-contained module
- Add Module Manager for install/uninstall
- Create module_registry database table
- Support hot-reloading of modules
- Move data imports into invcount module
- Update all templates and routes to new structure

Version bumped to 0.16.0

templates/admin_dashboard.html

@@ -4,28 +4,45 @@
 
 {% block content %}
 <div class="dashboard-container">
-<div class="dashboard-header" style="margin-top: var(--space-lg);">
+    <div class="dashboard-header" style="margin-top: var(--space-lg);">
         <div class="header-left" style="display: flex; align-items: center; gap: var(--space-md);">
             <a href="{{ url_for('home') }}" class="btn btn-secondary btn-sm">
                 <i class="fa-solid fa-arrow-left"></i> Back to Home
             </a>
             <h1 class="page-title" style="margin-bottom: 0;">Admin Dashboard</h1>
         </div>
+        <div class="header-right">
+            <a href="{{ url_for('module_manager_ui') }}" class="btn btn-primary btn-sm">
+                <i class="fa-solid fa-puzzle-piece"></i> Module Manager
+            </a>
+        </div>
     </div>
 
     <div class="modules-section">
         <h2 class="section-title">Modules</h2>
+        
+        {% if modules %}
         <div class="modules-grid">
-            <a href="{{ url_for('counting.admin_dashboard') }}" class="module-card">
-                <div class="module-icon">📊</div> <h3 class="module-name">Counts</h3>
-                <p class="module-desc">Cycle counts & physical inventory</p>
-            </a>
-            
-            <a href="{{ url_for('cons_sheets.admin_processes') }}" class="module-card module-card-link">
-                <div class="module-icon">📝</div> <h3 class="module-name">Consumption Sheets</h3>
-                <p class="module-desc">Production consumption tracking</p>
+            {% for module in modules %}
+            <a href="/{{ module.module_key }}/admin" class="module-card module-card-link">
+                <div class="module-icon">
+                    {% if module.icon %}
+                        <i class="{{ module.icon }}"></i>
+                    {% else %}
+                        📦
+                    {% endif %}
+                </div>
+                <h3 class="module-name">{{ module.module_name }}</h3>
+                <p class="module-desc">{{ module.description }}</p>
             </a>
+            {% endfor %}
         </div>
+        {% else %}
+        <div class="alert alert-info">
+            <i class="fa-solid fa-info-circle"></i> No modules installed yet.
+            <a href="{{ url_for('module_manager_ui') }}">Install modules</a> to get started.
+        </div>
+        {% endif %}
     </div>
 </div>
 {% endblock %}