feat(devices): Open Pixel Control (OPC) target type

Adds support for Open Pixel Control receivers (Fadecandy boards,
xLights/Falcon endpoints, OPC bridges, art-installation controllers,
hobbyist LED driver software). OPC is a tiny TCP protocol on port
7890 with a 4-byte header [channel][cmd][len_hi][len_lo] + RGB body.

Backend:
- OPCClient opens one persistent TCP connection and streams frames as
  header+body byte pairs. Channel 0 broadcasts to every output on the
  OPC server; channels 1-255 address a specific channel on multi-output
  servers (Fadecandy with multiple Open Pixel chains).
- supports_fast_send=True with a synchronous send_pixels_fast hot path.
  The fast path skips the async drain so the OS write-buffer flushes
  on its own schedule -- exactly what ambilight streaming wants.
- Brightness applies client-side before the frame is sent (OPC has no
  reply channel for hardware-side brightness).
- Health check opens a TCP connection and closes it.
- OPCConfig joins the typed config union; storage gains an opc_channel
  field; full to_dict/from_dict/to_config wiring.
- 36 unit tests cover URL parsing, header construction, send_pixels
  emitting header+body in order, brightness application, list and
  flat-array input shapes, drain behavior, connection lifecycle,
  provider validate/discover/capabilities, Device.to_config round-trip.

Frontend:
- 'opc' in DEVICE_TYPE_KEYS (next to 'ddp'), paper-plane icon -- same
  as DDP since both are open pixel-streaming protocols.
- isOpcDevice predicate + per-type field show/hide.
- Optional channel number input (default 0 = broadcast) with hint copy
  explaining the channel semantics.
- Locale strings in en/ru/zh.

No native discovery (OPC has no discovery protocol); users supply
the receiver IP manually.

TODO.md

@@ -738,7 +738,10 @@ After phase 1 the codebase will have 3 fresh examples of "ping the LAN, listen f
 
 ### Phase 5 — Open pixel protocols (cheap completionism)
 
-- [ ] OPC (Open Pixel Control)
+- [x] **OPC (Open Pixel Control)** — TCP, port 7890, 4-byte header
+      `[channel][cmd][len_hi][len_lo]` + RGB body. Channel 0 broadcasts.
+      Single-pixel-strip protocol, no discovery, no pairing. 36 unit
+      tests. Fadecandy + xLights + hobbyist receivers reachable.
 - [ ] TPM2.net
 
 ### Phase 6 — PC gaming RGB completion