fix(tui): capture composer padding before prompt

Add an explicit mouse capture cell over the left padding before the prompt glyph. Drags starting there now delegate to the composer input at offset 0 instead of starting terminal-level selection over the prompt chrome.

Made-with: Cursor

ui-tui/src/components/appLayout.tsx

@@ -205,6 +205,16 @@ const ComposerPane = memo(function ComposerPane({
             ))}
 
             <Box position="relative">
+              <Box
+                height={inputHeight}
+                left={-1}
+                onMouseDown={captureInputDrag}
+                onMouseDrag={dragIntoInput}
+                onMouseUp={() => inputMouseRef.current?.end()}
+                position="absolute"
+                width={1}
+              />
+
               <Box onMouseDown={captureInputDrag} onMouseDrag={dragIntoInput} onMouseUp={() => inputMouseRef.current?.end()} width={pw}>
                 {sh ? (
                   <Text color={ui.theme.color.shellDollar}>$ </Text>