main: code-review fixes — brightness units, OTA stall cap, stats + locking

- display: s_last_pwm cached the raw 0-100 percentage at init but
  display_wake writes it straight to REG_PWM (0-255 duty) — first wake
  ran visibly dim until a /config brightness change. Shared pct_to_duty
  helper now converts in both paths.
- http_api: cap consecutive OTA recv timeouts (a stalled client spun
  forever holding s_ota_in_progress, wedging OTA until reboot); cap
  viewport name at 54 chars so viewport-<name> fits the 63-byte mDNS
  label; log mdns_service_refresh failures; /state builds JSON from a
  snapshot instead of holding the state lock across ~25 cJSON allocs;
  respond_400 delegates to respond_status.
- stream_server: bytes_in_window now counts painted frames only (frames
  discarded while asleep inflated the first post-wake window's MB/s,
  avg-jpeg and chunk/wire averages; recv_bytes folded in); so_rcvbuf
  carried into the stats snapshot under the mux; drop dead HEADER_BYTES;
  merge read_body_instrumented into read_n.
- state_machine: transition mutex serializes concurrent wake/sleep
  (display side-effects ran after the state lock dropped, so racing
  callers could leave the backlight contradicting st->state); wake-path
  placeholder paint now takes the decoder lock like the stream path
  (concurrent esp_lcd_panel_draw_bitmap from two tasks isn't safe).
- local_screens: overlay paint takes the decoder lock too; check the
  overlay timer create; panel dims from viewport_state.h.
- jpeg_decoder: try_lock before init returns busy instead of passing a
  NULL semaphore to xSemaphoreTake.
- net_eth: clear cached IP string on link down (stale /state + info).
- dead code: display_present_bgr888, TOUCH_FT5426_ADDR, touch s_task,
  JPEG_DECODER_MAX_OUTPUT_BYTES; doc drift in nvs_config.h /
  jpeg_decoder.h / app_main flag legend.

1 files changed, 3 insertions, 0 deletions

diff --git a/main/net_eth.c b/main/net_eth.c
index 28ef782..3b4a76c 100644
--- a/main/net_eth.c
+++ b/main/net_eth.c
@@ -53,6 +53,9 @@ static void on_eth_event(void *arg, esp_event_base_t event_base,
     case ETHERNET_EVENT_DISCONNECTED:
         ESP_LOGW(TAG, "link down");
         xEventGroupClearBits(s_event_group, BIT_GOT_IP);
+        // Clear the cached address so /state and the info screen don't
+        // keep reporting a lease we no longer hold.
+        s_ip_str[0] = '\0';
         break;
     case ETHERNET_EVENT_START:
         ESP_LOGI(TAG, "ethernet started");