Tidy: delete dead code, inline single-use helpers, fix double-lock

Code-review pass after the M5 colour fix. -126 net lines (-215 / +89).

Dead code removed:
- display_fill, display_test_pattern — M3 bring-up self-tests, no callers
- net_eth_is_up — no callers
- nvs_config_reset — no callers (factory-reset gesture was removed earlier;
  NVS wipe now goes through `idf.py erase-flash`)

Helpers inlined or collapsed (each had one call site):
- state_name / orientation_name — ternaries at the cJSON site
- fmt_bytes / fmt_uptime — inline scope in local_screens_show_info
- expected_dims (http_api) + effective_dims (local_screens) — merged into
  viewport_state_effective_dims() in viewport_state, one canonical source

state_machine cleanup:
- arm_idle_timer_unlocked / disarm_idle_timer collapsed to one arm_idle_timer(ms)
  that takes the snapshotted timeout (ms==0 disables). Removes the misleading
  name and the second viewport_state lock acquisition per painted frame.
- state_machine_set + state_machine_frame_painted now snapshot state + idle_ms
  under one lock.

mdns_service cleanup:
- snapshot_state / apply_hostname / apply_txt collapsed into a single
  apply_state(include_hostname) helper; mdns_service_start grabs hostname
  once for hostname_set + log instead of going under the lock three times.

http_api cleanup:
- POST /config brightness-changed path uses the local `bright` it already
  validated instead of re-locking + re-reading st->brightness.
- Trimmed verbose bring-up comments (stack-size justification, M5 saga,
  DSI shadow struct) to one line each — kept the load-bearing facts.

Smoke-tested on hardware: boot clean, display + JPEG + touch all up
([DJT]), info screen renders, no crashes.

1 files changed, 11 insertions, 14 deletions

diff --git a/main/state_machine.c b/main/state_machine.c
index a4d4dab..0a6d491 100644
--- a/main/state_machine.c
+++ b/main/state_machine.c
@@ -12,21 +12,15 @@ static const char *TAG = "state";
 
 static esp_timer_handle_t s_idle_timer;
 
-static void arm_idle_timer_unlocked(void)
+// ms = idle_timeout_ms snapshotted under the caller's state lock.
+// ms == 0 disables the timer (caller's idle_timeout_ms feature).
+static void arm_idle_timer(uint32_t ms)
 {
-    viewport_state_lock();
-    uint32_t ms = viewport_state_get()->idle_timeout_ms;
-    viewport_state_unlock();
-    if (ms == 0) return;
     esp_timer_stop(s_idle_timer);
+    if (ms == 0) return;
     esp_timer_start_once(s_idle_timer, (uint64_t)ms * 1000ULL);
 }
 
-static void disarm_idle_timer(void)
-{
-    esp_timer_stop(s_idle_timer);
-}
-
 static void idle_timer_fired(void *arg)
 {
     ESP_LOGI(TAG, "idle timer expired — sleeping");
@@ -56,6 +50,7 @@ esp_err_t state_machine_set(viewport_run_state_t target)
     }
     st->state = target;
     bool configured = st->configured;
+    uint32_t idle_ms = st->idle_timeout_ms;
     viewport_state_unlock();
 
     if (target == VIEWPORT_STATE_AWAKE) {
@@ -67,10 +62,10 @@ esp_err_t state_machine_set(viewport_run_state_t target)
             if (configured) local_screens_show_loading();
             else            local_screens_show_info();
         }
-        arm_idle_timer_unlocked();
+        arm_idle_timer(idle_ms);
         ESP_LOGI(TAG, "AWAKE (%s)", configured ? "configured" : "no scrypted URL");
     } else {
-        disarm_idle_timer();
+        arm_idle_timer(0);
         if (display_is_up()) display_sleep();
         ESP_LOGI(TAG, "ASLEEP");
     }
@@ -80,9 +75,11 @@ esp_err_t state_machine_set(viewport_run_state_t target)
 void state_machine_frame_painted(void)
 {
     viewport_state_lock();
-    bool awake = (viewport_state_get()->state == VIEWPORT_STATE_AWAKE);
+    viewport_state_t *st = viewport_state_get();
+    bool awake = (st->state == VIEWPORT_STATE_AWAKE);
+    uint32_t idle_ms = st->idle_timeout_ms;
     viewport_state_unlock();
-    if (awake) arm_idle_timer_unlocked();
+    if (awake) arm_idle_timer(idle_ms);
 }
 
 void state_machine_set_local(viewport_run_state_t target)