feat(devices): Govee LAN target type

Adds support for Govee Wi-Fi smart bulbs and ambient-lighting kits via
their LAN API (opened in 2023). Discovery is multicast UDP on
239.255.255.250:4001; control commands go unicast to the device's port
4003; responses arrive on port 4002.

Each device requires "LAN Control" toggled ON in the Govee Home app
(Device -> settings -> LAN Control). Devices with LAN Control disabled
silently fail to appear in discovery and won't respond to commands; the
UI hint copy reminds users.

Backend:
- GoveeClient is a single-pixel UDP adapter: averages the strip to one
  RGB triple and pushes a 'colorwc' command with colorTemInKelvin=0 to
  select pure RGB mode (non-zero kelvin would switch the bulb to CCT
  mode and ignore the RGB values).
- Brightness folds into the RGB scaling so we burn one packet per
  frame instead of two.
- supports_fast_send=True with a synchronous send_pixels_fast hot path.
  Default rate gate 50 ms (~20 Hz); UDP fire-and-forget tolerates it.
- Multicast discovery: scan request to 239.255.255.250:4001, listen on
  port 4002, parse the inner data dict for IP + device-id + SKU +
  firmware version. Degrades to [] when port 4002 is already bound or
  network is unavailable.
- Health check sends devStatus and waits 1.5s for any reply; the error
  message points at the LAN-Control toggle since that's the #1 root
  cause of silent failures.
- GoveeConfig joins the typed config union; storage gains
  govee_min_interval_ms; full to_dict/from_dict/to_config wiring.
- 40 unit tests cover URL parsing, scan-reply parsing (rejecting
  non-scan commands and malformed JSON), payload builders (colorwc
  with colorTemInKelvin=0, brightness clamping, power as 1/0 not
  true/false), strip averaging, rate limiting, fast-send hot path,
  provider validate/discover/health, Device.to_config round-trip.

Frontend:
- 'govee' in DEVICE_TYPE_KEYS (next to 'lifx'), lightbulb icon
  (deliberate smart-bulb family grouping).
- isGoveeDevice predicate + per-type field show/hide.
- Rate-limit number input (default 50 ms).
- URL hint copy explicitly instructs users to enable LAN Control in
  the Govee Home app -- the #1 source of "why isn't my Govee
  responding?" support churn.
- Locale strings in en/ru/zh.

TODO.md

@@ -727,7 +727,11 @@ After phase 1 the codebase will have 3 fresh examples of "ping the LAN, listen f
       conversion; broadcast discovery via GetService/StateService probe;
       47 unit tests. Single-pixel adapter shape, identical to WiZ
       structurally. Frontend wired via subagent.
-- [ ] Govee LAN API (2023+)
+- [x] **Govee LAN API** — UDP JSON on port 4003 (control) + 4002
+      (responses) + 4001 (multicast discovery on 239.255.255.250).
+      Single-pixel `colorwc` command with `colorTemInKelvin=0` for RGB
+      mode. **Per-device "LAN Control" toggle required in Govee Home
+      app.** 40 unit tests. Frontend wired via subagent.
 - [ ] Twinkly
 - [ ] Nanoleaf OpenAPI
 - [ ] Mi-Light / MiBoxer UDP gateway