revert: remove ResizeObserver and stty-on-every-resize to fix infinite loop

The ResizeObserver detected size changes caused by the stty command
output appearing in the terminal, creating an infinite resize loop:
1. Resize detected -> fit() -> send resize to backend
2. Backend sends stty command through PTY
3. stty text appears in terminal output
4. ResizeObserver detects content height change
5. fit() calculates new rows -> send resize
6. Loop continues forever

Reverted to:
- Window resize event instead of ResizeObserver
- stty command only sent once on first resize

This means the container shell stays at the initial size and won't
dynamically resize when the browser window changes, but prevents
the infinite loop.

apps/api/src/services/terminal_session.py

@@ -142,11 +142,6 @@ class TerminalSession:
         if self._closed:
             logger.warning("Cannot resize: session is closed")
             return
-        
-        # Only resize if dimensions actually changed
-        if self._cols == cols and self._rows == rows:
-            return
-            
         self._cols = cols
         self._rows = rows
         logger.info(f"resize() called for session {self.session_id}: {cols}x{rows}")
@@ -154,9 +149,12 @@ class TerminalSession:
         
         # Docker exec doesn't forward PTY resize to the container process,
         # so we need to explicitly set the size inside the container shell.
-        stty_cmd = f"stty cols {cols} rows {rows}\n".encode()
-        await self.write_input(stty_cmd)
-        logger.info(f"Sent stty command to container for session {self.session_id}: {cols}x{rows}")
+        # Only do this on the first resize to avoid interfering with user input.
+        if not getattr(self, '_stty_sent', False):
+            self._stty_sent = True
+            stty_cmd = f"stty cols {cols} rows {rows}\n".encode()
+            await self.write_input(stty_cmd)
+            logger.info(f"Sent stty command to container for session {self.session_id}: {cols}x{rows}")
 
     async def reset(self) -> None:
         """Reset the session by killing the process and clearing state."""