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, 31 insertions, 0 deletions

diff --git a/main/mint_health.h b/main/mint_health.h
new file mode 100644
index 0000000..f047d6a
--- /dev/null
+++ b/main/mint_health.h
@@ -0,0 +1,31 @@
+#ifndef MINT_HEALTH_H
+#define MINT_HEALTH_H
+
+#include "esp_err.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+#define MINT_HEALTH_MAX             8
+#define MINT_HEALTH_PROBE_INTERVAL_S  300
+#define MINT_HEALTH_PROBE_TIMEOUT_MS  15000
+#define MINT_HEALTH_RECOVERY_THRESHOLD 3
+
+typedef struct {
+    char url[256];
+    bool reachable;
+    uint8_t consecutive_successes;
+    int64_t last_probe_ms;
+    int last_http_status;
+} mint_status_t;
+
+typedef void (*mint_health_changed_cb)(void);
+
+esp_err_t mint_health_init(const char urls[][256], int count);
+void mint_health_start(void);
+void mint_health_stop(void);
+const mint_status_t *mint_health_get_all(int *out_count);
+bool mint_health_is_reachable(const char *url);
+void mint_health_mark_unreachable(const char *url);
+void mint_health_register_callback(mint_health_changed_cb cb);
+
+#endif