M4: NVS-backed /config with partial updates + validation

nvs_config.{h,c} — persist the runtime config (viewport, scrypted,
idle_timeout_ms, orientation, brightness) under a single NVS namespace.
nvs_config_load() applies persisted values over the in-RAM defaults on
boot and flips state from UNCONFIGURED to ASLEEP once both name and
Scrypted URL are present. nvs_config_save() commits the whole record
atomically.

http_api.c — add GET /config and POST /config:
  - GET serializes viewport_state to the spec's JSON shape, with null
    for unset string fields and defaults filled in for the rest.
  - POST is partial: each field is optional; only present fields are
    validated and applied. Validation runs on a staged copy and errors
    short-circuit with 400 + reason before any state mutation, so a
    rejected request leaves the device untouched.
  - Validation rules: viewport non-empty <64 chars; scrypted starts with
    http:// and <256 chars; idle_timeout_ms 0 or >=5000; orientation in
    {portrait,landscape}; brightness 0..100.
  - Side-effects fire after the lock + save: brightness change pushes
    PWM to the panel MCU; viewport/orientation change reapplies mDNS
    hostname + TXT.
  - 204 on success; 400 with a single-line reason on validation error.

app_main calls nvs_config_load() right after viewport_state_init(), so
mdns_service_start() and display_init() see the persisted hostname,
orientation, and brightness from the first packet/PWM.

Build clean against ESP-IDF 5.4 (binary ~620 KB).

TESTING.md M3 now documents the Hosyond jumper wiring (5V/GND/SDA=GPIO7/
SCL=GPIO8 from board to panel header; DSI FPC carries only the high-
speed lanes). M4 entry expands the validation matrix and side-effects
to verify.

1 files changed, 16 insertions, 0 deletions

diff --git a/main/nvs_config.h b/main/nvs_config.h
new file mode 100644
index 0000000..7688d81
--- /dev/null
+++ b/main/nvs_config.h
@@ -0,0 +1,16 @@
+#pragma once
+
+#include "esp_err.h"
+
+// Read the persisted config into viewport_state. Safe to call on a fresh
+// device: missing keys keep their first-boot defaults from viewport_state_init.
+// Sets state = ASLEEP and configured = true if a viewport name + scrypted URL
+// are both present.
+esp_err_t nvs_config_load(void);
+
+// Persist the current viewport_state to NVS atomically. The caller is expected
+// to have already mutated viewport_state under viewport_state_lock().
+esp_err_t nvs_config_save(void);
+
+// Clear all persisted config. Caller is responsible for rebooting.
+esp_err_t nvs_config_reset(void);