feat: multi-mint wallet with health tracking, WPA auto-detect, display gating

Squash merge of feature/multi-mint-support (21 commits):

Multi-mint wallet:
- Accept payments from 4 mints: minibits, coinos, 21mint, lnvoltz
- Periodic health probing (300s interval, 3 recovery threshold)
- Multi-wallet init with nucula_wallet_init_multi()
- /mints and /wallet API endpoints

WPA auto-detect:
- wifi_auth_mode config field (default WPA2, supports WPA3)
- Runtime mapping to wifi_auth_mode_t in STA config

Display gating:
- display_enabled config field (default true)
- Guards display_init/display_update per-board

Bug fixes:
- 3s delay before service start prevents lwip mem_free assertion
- Real npub in discovery (identity_get()->npub_hex)
- Health probe interval 300s (production value)
- Duplicate services_start_task call removed
- UTF-8 arrow replaced with ASCII in log message

Tests: 61+14 unit tests passing, firmware builds clean

1 files changed, 7 insertions, 0 deletions

diff --git a/tests/unit/stubs/freertos/semphr.h b/tests/unit/stubs/freertos/semphr.h
new file mode 100644
index 0000000..0389b11
--- /dev/null
+++ b/tests/unit/stubs/freertos/semphr.h
@@ -0,0 +1,7 @@
+#ifndef STUBS_FREERTOS_SEMPHR_H
+#define STUBS_FREERTOS_SEMPHR_H
+
+#include "FreeRTOS.h"
+#include "task.h"
+
+#endif