feat(tui): add multiline chat input with dynamic height

- Support Shift+Enter to insert newlines in chat input
- Chat input container expands dynamically up to 8 lines
- Enter key sends message as before
- Fix cursor line background to match unselected lines

strix/interface/assets/tui_styles.tcss

@@ -117,7 +117,7 @@ Screen {
     margin-right: 0;
     padding: 0;
     layout: horizontal;
-    align-vertical: middle;
+    align-vertical: top;
 }
 
 #chat_input_container:focus-within {
@@ -134,7 +134,7 @@ Screen {
     height: 100%;
     padding: 0 0 0 1;
     color: #737373;
-    content-align-vertical: middle;
+    content-align-vertical: top;
 }
 
 #chat_history:focus {
@@ -144,7 +144,7 @@ Screen {
 #chat_input {
     width: 1fr;
     height: 100%;
-    background: #121212;
+    background: transparent;
     border: none;
     color: #d4d4d4;
     padding: 0;
@@ -155,6 +155,14 @@ Screen {
     border: none;
 }
 
+#chat_input .text-area--cursor-line {
+    background: transparent;
+}
+
+#chat_input:focus .text-area--cursor-line {
+    background: transparent;
+}
+
 #chat_input > .text-area--placeholder {
     color: #525252;
     text-style: italic;

strix/interface/tui.py

@@ -55,7 +55,15 @@ class ChatTextArea(TextArea):  # type: ignore[misc]
     def set_app_reference(self, app: "StrixTUIApp") -> None:
         self._app_reference = app
 
+    def on_mount(self) -> None:
+        self._update_height()
+
     def _on_key(self, event: events.Key) -> None:
+        if event.key == "shift+enter":
+            self.insert("\n")
+            event.prevent_default()
+            return
+
         if event.key == "enter" and self._app_reference:
             text_content = str(self.text)  # type: ignore[has-type]
             message = text_content.strip()
@@ -69,6 +77,20 @@ class ChatTextArea(TextArea):  # type: ignore[misc]
 
         super()._on_key(event)
 
+    @on(TextArea.Changed)  # type: ignore[misc]
+    def _update_height(self, _event: TextArea.Changed | None = None) -> None:
+        if not self.parent:
+            return
+
+        line_count = self.document.line_count
+        target_lines = min(max(1, line_count), 8)
+
+        new_height = target_lines + 2
+
+        if self.parent.styles.height != new_height:
+            self.parent.styles.height = new_height
+            self.scroll_cursor_visible()
+
 
 class SplashScreen(Static):  # type: ignore[misc]
     PRIMARY_GREEN = "#22c55e"