Fix ESP32 audio dropouts and improve streaming reliability

FLAC encoder now tracks frame numbers and reuses per-channel sample
buffers to reduce allocations. Client dialer always frees the active
slot on release so mDNS re-emissions can reconnect cleanly. Server
send buffer uses drop-oldest instead of hard failure, with lateness
tracking. Added engine stats logging (chunks/sec, kbps) and writer
diagnostics. UI simplified to a single play/pause toggle that
auto-starts playback on preset selection. Added --preset CLI flag
for headless startup. Removed stale systemd service files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile.cross

@@ -20,6 +20,7 @@ RUN dpkg --add-architecture armhf \
 
 WORKDIR /src
 COPY go.mod go.sum ./
+COPY third_party/sendspin-go/go.mod third_party/sendspin-go/go.sum ./third_party/sendspin-go/
 RUN go mod download
 
 COPY . .