fix: auto-proceed installer warnings in headless mode

`SetupLog.wait_continue()` previously blocked on an asyncio Event that
could only be set by the Textual `enter` key binding. When the installer
runs with `--headless`, Textual has no terminal-attached UI to receive
keystrokes, so the WSL/LiveCD environment warning prompt would hang
forever waiting for an Enter press that never arrives.

Detect headless mode and auto-continue after printing the warning so
`./backend.ai install --headless --non-interactive ...` proceeds without
human input.

Author: achimnol

changes/11564.fix.md

@@ -0,0 +1 @@
+Fix the installer hanging on the WSL/LiveCD environment warning when run with `--headless` because the "Press Enter to continue" prompt could not receive keystrokes; in headless mode it now auto-proceeds after printing the warning.

src/ai/backend/install/widgets.py

@@ -97,7 +97,12 @@ async def wait_continue(self) -> None:
         """
         Block until the user concludes the dialog.
         If the user cancels, the result will be None.
+        In headless mode there is no terminal-attached UI to receive the Enter
+        keypress, so we auto-proceed instead of blocking forever.
         """
+        if self.app.is_headless:
+            self.write(Text.from_markup("\n[dim](headless mode: auto-continuing)[/]\n"))
+            return
         self.write(Text.from_markup("\nPress [bold]Enter[/] to continue...\n"))
         self._continue.clear()
         await self._continue.wait()