fix(pi3): auto-focus terminal + log serial input bytes

PiTerminal didn't call term.focus() on mount, so xterm.js stayed
passive — onData only fires when the DOM element has focus.  Users
saw the boot prompt but their keystrokes went to whatever element
held focus when they clicked Run (canvas, code editor), never
reaching the bridge.  Calling focus() right after fit() makes the
prompt receive input the moment it's visible.

The qemu_manager change adds INFO-level logging when serial_input
WebSocket messages reach send_serial_bytes — useful diagnostic for
future Pi3 input problems (proves whether bytes reached the backend
before we look at TTY / kernel / PL011 wiring).

backend/app/services/qemu_manager.py

@@ -118,12 +118,18 @@ class QemuManager:
 
     async def send_serial_bytes(self, client_id: str, data: bytes) -> None:
         inst = self._instances.get(client_id)
-        if inst and inst._serial_writer:
-            inst._serial_writer.write(data)
-            try:
-                await inst._serial_writer.drain()
-            except Exception as e:
-                logger.warning('send_serial_bytes drain: %s', e)
+        if not inst:
+            logger.warning('send_serial_bytes: no instance for client_id=%s', client_id)
+            return
+        if not inst._serial_writer:
+            logger.warning('send_serial_bytes: %s has no serial writer (qemu not connected yet?)', client_id)
+            return
+        logger.info('send_serial_bytes: %s sending %d bytes: %r', client_id, len(data), bytes(data[:32]))
+        inst._serial_writer.write(data)
+        try:
+            await inst._serial_writer.drain()
+        except Exception as e:
+            logger.warning('send_serial_bytes drain: %s', e)
 
     # ── Boot sequence ─────────────────────────────────────────────────────────
 

frontend/src/components/raspberry-pi/PiTerminal.tsx

@@ -48,6 +48,11 @@ export const PiTerminal: React.FC<PiTerminalProps> = ({ boardId }) => {
       } catch (_) {
         /* ignore if dimensions not ready */
       }
+      // Without an explicit focus call xterm.js stays passive — onData
+      // only fires when the DOM element has focus, so users staring at
+      // a working prompt see no echo because their keystrokes go to
+      // whatever element had focus at mount time (canvas, code editor).
+      try { term.focus(); } catch (_) { /* container may not be visible */ }
     });
 
     termRef.current = term;