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

static/css/04-interactive/_chat.css

@@ -783,12 +783,13 @@
    ========================================================================== */
 
 @media (max-width: 480px) {
+    /* Default compact: bottom sheet */
     .chat-panel {
         bottom: 0;
         left: 0;
         right: 0;
         width: 100%;
-        max-height: 70vh;
+        max-height: 55vh;
         border-radius: 8px 8px 0 0;
     }
 
@@ -798,6 +799,75 @@
     }
 
     .chat-messages {
-        max-height: 200px;
+        max-height: 180px;
+    }
+
+    /* Hide desktop-only layout modes on mobile */
+    .chat-mode-btn[data-mode="chat-half"],
+    .chat-mode-btn[data-mode="chat-float"],
+    .chat-mode-btn[data-mode="chat-full"] {
+        display: none;
+    }
+
+    /* Show mobile split button */
+    .chat-mode-btn[data-mode="chat-split"] {
+        display: flex;
+    }
+
+    /* Mobile split: CV on top, chat on bottom half */
+    .chat-panel.chat-split {
+        bottom: 0;
+        left: 0;
+        right: 0;
+        top: auto;
+        width: 100%;
+        height: 50vh;
+        max-height: 50vh;
+        border-radius: 8px 8px 0 0;
+        border-top: 2px solid var(--accent-green, #27ae60);
+    }
+
+    .chat-panel.chat-split .chat-messages {
+        max-height: none;
+        flex: 1;
+    }
+
+    /* Force compact on mobile for desktop-only modes (safety) */
+    .chat-panel.chat-half,
+    .chat-panel.chat-float,
+    .chat-panel.chat-full {
+        bottom: 0;
+        left: 0;
+        right: 0;
+        top: auto;
+        width: 100%;
+        height: auto;
+        max-height: 55vh;
+        border-radius: 8px 8px 0 0;
+        border: 1px solid var(--border-light, #e0e0e0);
+        resize: none;
+    }
+
+    /* Tooltips: prevent overflow on mobile */
+    .chat-mode-btn[title]:hover::after {
+        display: none;
+    }
+
+    /* Tighter header on mobile */
+    .chat-header {
+        padding: 8px 10px;
+        font-size: 0.8rem;
+    }
+
+    .chat-mode-btn {
+        font-size: 0.85rem;
+        padding: 3px;
+    }
+}
+
+/* Hide mobile split button on desktop */
+@media (min-width: 481px) {
+    .chat-mode-btn[data-mode="chat-split"] {
+        display: none;
     }
 }