Fix direct-write upload hanging on stall or error frame (MJ-19)

[balloob]

Problem

The BLE direct-write flow in httpdocs/js/ble-common.js had no ACK timeout and ignored mid-stream error frames. As a result a stalled transfer or a firmware error frame left the UI stuck at "Uploading N%" forever, with onComplete/onError never firing. Because directWriteState.active stayed true, a subsequent upload was also blocked by "Direct write already in progress".

Two concrete gaps:

• The chunk-sending path (sendNextDirectWriteChunk / _activateFullDirectWrite) had no timer, so silence from the device hung indefinitely.
• handleDirectWriteNotification only recognized a NACK while awaitingPartialStart, so a mid-stream {0xFF,0x71,err,0x00} NACK or the 2-byte {0xFF,0xFF} compressed-error frame matched no pattern → pendingAcks never drained.

Fix

• Add a per-ACK timeout (DIRECT_WRITE_ACK_TIMEOUT_MS = 8000, matching the existing config-write ack timeout) armed when sending the start command, chunks, and the end command, and re-armed on each recognized ack. On expiry the upload aborts via the existing _abortDirectWrite path, firing onComplete(false, err) and clearing directWriteState. The timer is cleared once the end command acks (0x72), so the display-refresh phase remains bounded by the firmware's own refresh timeout (0x74) rather than the short ack timer.
• Make handleDirectWriteNotification recognize mid-stream error frames at any point in the flow: a 4-byte {0xFF,cmd,err,0x00} NACK for any direct-write opcode, and the 2-byte {0xFF,0xFF} frame. Both fail the operation cleanly with a descriptive message. The existing partial-start NACK fallback-to-full behavior is preserved.

The change is scoped to the direct-write send/notification path; encoding/image code is untouched.

Verification

• node --check httpdocs/js/ble-common.js passes.
• Drove handleDirectWriteNotification in a small Node/VM harness: a {0xFF,0xFF} frame and a mid-stream {0xFF,0x71,err,0x00} NACK each return true, fire onComplete(false, err) once with a descriptive message, and reset directWriteState to null. Confirmed the partial-start NACK still falls back to full (no hard abort), and that the ack-timeout path aborts a stalled transfer and cleans up state.