fix(ui): keep the input box usable while a response is streaming

While a turn was in flight (isThinking or streamingText), the ChatInput was
unmounted and replaced with a static "Waiting for response…" box — so the user
couldn't type ahead during output, the input field just vanished.

Now ChatInput stays mounted the whole time: the user can type and edit while
output streams in. A one-line status above it ("Waiting… esc to interrupt — type
ahead, Enter sends when done.") replaces the old full-width box. Submitting a
second turn mid-stream is still prevented — handleSubmit now guards on
isThinking/streamingText and returns early, so Enter is a no-op until the
in-flight turn finishes (the typed text stays in the box). The original race this
swap was protecting against (overlapping turns on the same session) is preserved
by that guard.

Verified: typecheck clean, build 274.76 KB, 301 tests pass.

src/ui/ink/App.tsx

@@ -776,6 +776,11 @@ export function App({
   }
 
   async function handleSubmit(raw: string) {
+    // A turn is already in flight (streaming output or waiting for the first chunk). Keep the
+    // input box mounted so the user can type ahead, but ignore Enter so a second turn can't
+    // overlap onto the same session. The in-flight turn is the source of truth; the typed text
+    // stays in the box for when the turn finishes.
+    if (isThinking || streamingText !== null) return;
     setInputValue("");
     const trimmed = raw.trim();
     if (!trimmed) return;
@@ -1198,25 +1203,18 @@ export function App({
                   gitInfo={gitInfo}
                 />
               )}
-              {isThinking || streamingText !== null ? (
-                // Swapped in for ChatInput while a turn is in flight. Closes a real (if rare) race:
-                // handleSubmit had no guard against firing while a turn was already running, so
-                // typing and hitting Enter mid-stream could submit a second overlapping turn onto
-                // the same session.
-                <Box borderStyle="round" borderColor={ACCENT_HEX} paddingX={1} width="100%">
-                  <Text dimColor>Waiting for response… (esc to interrupt)</Text>
-                </Box>
-              ) : (
-                <ChatInput
-                  value={inputValue}
-                  onChange={setInputValue}
-                  onSubmit={handleSubmit}
-                  cwd={cwd}
-                  history={history}
-                  availableColumns={chatColumns}
-                  isActive={!filePanelFocused}
-                />
+              {(isThinking || streamingText !== null) && (
+                <Text dimColor>  Waiting for response… (esc to interrupt) — type ahead, Enter sends when done.</Text>
               )}
+              <ChatInput
+                value={inputValue}
+                onChange={setInputValue}
+                onSubmit={handleSubmit}
+                cwd={cwd}
+                history={history}
+                availableColumns={chatColumns}
+                isActive={!filePanelFocused}
+              />
             </>
           )}
         </Box>