feat: lazy load ninja-keys + HTML Invoker Commands API

- Lazy load ninja-keys only on CMD+K press (0 requests on initial load)
- Use esm.sh bundled module (3 requests vs ~81 previously)
- Add esm.sh to CSP whitelist
- Implement HTML Invoker Commands API for modals:
  - commandfor="modal-id" + command="show-modal" for opening
  - commandfor="modal-id" + command="close" for closing
  - Removes need for onclick handlers on modal buttons
- Refactor index.html into layout partials (head, body-scripts)
- Add comprehensive tests for both features

templates/partials/layout/head-scripts.html

@@ -0,0 +1,34 @@
+{{define "head-scripts"}}
+    <!-- Device Detection - Detect real mobile devices vs desktop browser -->
+    <script src="/static/js/device-detection.js"></script>
+
+    <!-- HTMX with SRI (Subresource Integrity) -->
+    <script src="https://unpkg.com/htmx.org@1.9.10"
+            integrity="sha384-D1Kt99CQMDuVetoL1lrYwg5t+9QdHe7NLX/SoJYkXDFfX37iInKRy5xLSi8nO7UC"
+            crossorigin="anonymous"></script>
+
+    <!-- Hyperscript Functions - Must load BEFORE hyperscript library -->
+    <!-- NOTE: cv-functions.js removed - hyperscript def statements are globally available -->
+    <!-- ✅ NO def limit with latest hyperscript - organized by category -->
+    <script type="text/hyperscript" src="/static/hyperscript/utils._hs"></script>
+    <script type="text/hyperscript" src="/static/hyperscript/toggles._hs"></script>
+    <script type="text/hyperscript" src="/static/hyperscript/hover-sync._hs"></script>
+    <script type="text/hyperscript" src="/static/hyperscript/keyboard._hs"></script>
+    <script type="text/hyperscript" src="/static/hyperscript/zoom._hs"></script>
+    <script type="text/hyperscript" src="/static/hyperscript/pdf-modal._hs"></script>
+
+    <!-- Color Theme System (JavaScript - hyperscript had parsing issues with colons in strings) -->
+    <script src="/static/js/color-theme.js"></script>
+
+    <!-- NOTE: footer-buttons-interaction.js removed - moved to hyperscript on footer element -->
+    <!-- NOTE: scroll-at-bottom-handler.js removed - duplicate of handleScroll() in utils._hs -->
+
+    <!-- Hyperscript - Declarative event handling for enhanced interactivity -->
+    <script src="https://unpkg.com/hyperscript.org@0.9.14"></script>
+
+    <!-- Ninja Keys - Lazy loaded on CMD+K (see body-scripts for loader) -->
+
+    <!-- Iconify - Load synchronously for immediate rendering -->
+    <!-- Using unpkg CDN (more reliable than code.iconify.design) -->
+    <script src="https://cdn.jsdelivr.net/npm/iconify-icon@2.1.0/dist/iconify-icon.min.js"></script>
+{{end}}