Fixed Video Output and stabilised PPPP connection

[Django1982]

What’s fixed

• PPPP uploads are stable now: metadata frame no longer double-NUL-terminated; AABB replies read under a lock (no more Pipe/EOF races).
• Web upload /api/files/local supports print=false (upload-only) and surfaces clearer PPPP error codes.
• File transfer handshake validates reply codes/timeouts; only sends END when start_print=True.
• PPPP/WebSocket stability and the earlier video fixes remain intact.

Verification

• Upload-only via /api/files/local → 200.
• Print upload via /api/files/local → 200 (verified live on the printer).