feat: mobile-first chat layout — split mode, hidden desktop modes, 79 tests

Mobile (≤480px):
- Hide Side Panel, Floating, Full Screen buttons
- Show Split button (50vh vertical split, CV visible above)
- Compact mode: 55vh max bottom sheet
- Force desktop modes to compact if somehow activated
- Disable tooltips on mobile (overflow prevention)
- Tighter header padding

Desktop (>480px):
- Split button hidden, all modes available as before

Tests: 85-chat-mobile.test.mjs — 79 assertions across
iPhone SE (320x568), iPhone 14 (393x852), iPhone 14 Pro Max (430x932),
plus desktop sanity check

templates/partials/widgets/chat-widget.html

@@ -20,6 +20,9 @@
             <button class="chat-mode-btn active" data-mode="" title="{{if eq .Lang "es"}}Compacto{{else}}Compact{{end}}" onclick="setChatSize('')">
                 <iconify-icon icon="mdi:message-outline"></iconify-icon>
             </button>
+            <button class="chat-mode-btn" data-mode="chat-split" title="{{if eq .Lang "es"}}Mitad de pantalla{{else}}Half screen{{end}}" onclick="setChatSize('chat-split')">
+                <iconify-icon icon="mdi:arrow-split-horizontal"></iconify-icon>
+            </button>
             <button class="chat-mode-btn" data-mode="chat-half" title="{{if eq .Lang "es"}}Panel lateral{{else}}Side panel{{end}}" onclick="setChatSize('chat-half')">
                 <iconify-icon icon="mdi:page-layout-sidebar-right"></iconify-icon>
             </button>
@@ -216,7 +219,7 @@ document.addEventListener('htmx:afterRequest', function(event) {
 // Set chat panel size
 function setChatSize(size) {
     var panel = document.getElementById('chat-panel');
-    panel.classList.remove('chat-half', 'chat-full', 'chat-float');
+    panel.classList.remove('chat-half', 'chat-full', 'chat-float', 'chat-split');
     // Reset all inline styles from drag/resize
     panel.style.top = '';
     panel.style.left = '';