Add styles for chat header and model picker

src/styles.css

@@ -319,6 +319,133 @@ body { background: var(--bg); color: var(--text); font-family: ui-sans-serif, sy
   font-size: 13px;
 }
 
+.header--chat {
+  justify-content: space-between;
+}
+
+.header-main {
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+
+.header-title {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.header-actions {
+  flex-shrink: 0;
+  margin-left: auto;
+  display: flex;
+  align-items: center;
+}
+
+.model-picker {
+  position: relative;
+}
+
+.model-picker-toggle {
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+  max-width: min(320px, 42vw);
+  padding: 8px 12px;
+  border: 1px solid var(--border);
+  border-radius: 999px;
+  background: var(--input-bg);
+  color: var(--text);
+  cursor: pointer;
+}
+
+.model-picker-toggle:hover {
+  border-color: var(--accent);
+}
+
+.model-picker-toggle:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+.model-picker-label,
+.model-picker-option-label {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.model-picker-caret,
+.model-picker-status {
+  flex-shrink: 0;
+  color: var(--muted);
+}
+
+.model-picker-menu {
+  position: absolute;
+  top: calc(100% + 8px);
+  right: 0;
+  min-width: 260px;
+  max-width: min(360px, calc(100vw - 32px));
+  max-height: 320px;
+  overflow-y: auto;
+  padding: 6px;
+  border: 1px solid var(--border);
+  border-radius: 12px;
+  background: var(--panel);
+  box-shadow: 0 14px 32px rgba(0,0,0,.35);
+  z-index: 20;
+}
+
+.model-picker-option {
+  width: 100%;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 12px;
+  padding: 9px 10px;
+  border: 0;
+  border-radius: 10px;
+  background: transparent;
+  color: var(--text);
+  cursor: pointer;
+  text-align: left;
+}
+
+.model-picker-option:hover {
+  background: var(--hover-bg);
+}
+
+.model-picker-option.selected {
+  background: var(--active-bg);
+}
+
+.model-picker-empty {
+  padding: 10px;
+  color: var(--muted);
+  font-size: 13px;
+}
+
+@media (max-width: 720px) {
+  .header--chat {
+    align-items: flex-start;
+    flex-wrap: wrap;
+  }
+
+  .header-actions {
+    width: 100%;
+    margin-left: 0;
+    justify-content: flex-end;
+  }
+
+  .model-picker-toggle {
+    max-width: min(100%, 320px);
+  }
+}
+
 .chat {
   display: grid;
   grid-template-columns: 1fr minmax(auto, 1000px) 1fr;