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, 9 insertions, 2 deletions

diff --git a/main/viewport_state.c b/main/viewport_state.c
index 3ae8f81..7ddf737 100644
--- a/main/viewport_state.c
+++ b/main/viewport_state.c
@@ -13,8 +13,6 @@ void viewport_state_init(void)
 {
     memset(&s_state, 0, sizeof(s_state));
     s_state.configured      = false;
-    // State is always AWAKE or ASLEEP. UNCONFIGURED is reported via the
-    // `configured` flag, not as a state value. Boot is always asleep.
     s_state.state           = VIEWPORT_STATE_ASLEEP;
     s_state.brightness      = 80;
     s_state.idle_timeout_ms = 60000;
@@ -46,3 +44,12 @@ const char *viewport_state_resolution_str(void)
                ? "480x800"
                : "800x480";
 }
+
+void viewport_state_effective_dims(uint16_t *w, uint16_t *h)
+{
+    viewport_state_lock();
+    viewport_orientation_t o = s_state.orientation;
+    viewport_state_unlock();
+    if (o == VIEWPORT_ORIENTATION_PORTRAIT) { *w = 480; *h = 800; }
+    else                                    { *w = 800; *h = 480; }
+}