meshdevicefile: only pause source socket when HTTP write buffer fills (#7816)

Previously, every binary frame received from the agent triggered an unconditional pause() on the incoming WebSocket followed by a resume() from the res.write() callback. This is correct backpressure handling but it forces stop-and-wait on every chunk — each chunk must wait for the HTTP write callback before the next one can flow from the agent. Combined with the small chunk size emitted by the agent and any non-trivial RTT between agent and server, throughput is effectively capped around 2 Mbit/s on real WAN agents regardless of available bandwidth. Standard Node.js streaming pattern: only pause the source when res.write() returns false (write buffer is actually full) and resume on the next 'drain' event. Otherwise frames flow continuously and the TCP stack handles overload through its own backpressure. Measured on production deployment with 29 random Win10/Win11 agents across distinct customer networks: Before: ~250 KB/s (~2 Mbit/s) median After: ~2.7 MB/s (~22 Mbit/s) median (~11x improvement) Best: 3.4 MB/s P25-P75: 2.6-2.74 MB/s (very tight distribution) A single 60 MB transfer drops from ~4 minutes to ~22 seconds. This addresses the recurring "slow file transfer" reports tracked in issues such as #5347 and #3550.