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, 8 insertions, 2 deletions

diff --git a/main/cashu.c b/main/cashu.c
index ec0566c..da12ff9 100644
--- a/main/cashu.c
+++ b/main/cashu.c
@@ -1,5 +1,6 @@
 #include "cashu.h"
 #include "config.h"
+#include "mint_health.h"
 #include "esp_log.h"
 #include "esp_http_client.h"
 #include "cJSON.h"
@@ -267,6 +268,11 @@ bool cashu_is_mint_accepted(const char *mint_url)
 {
     if (!mint_url || mint_url[0] == '\0') return false;
     const tollgate_config_t *cfg = tollgate_config_get();
-    if (strstr(mint_url, cfg->mint_url) != NULL) return true;
-    return (strcmp(mint_url, cfg->mint_url) == 0);
+    for (int i = 0; i < cfg->accepted_mint_count; i++) {
+        if (strstr(mint_url, cfg->accepted_mints[i]) != NULL ||
+            strcmp(mint_url, cfg->accepted_mints[i]) == 0) {
+            return mint_health_is_reachable(mint_url);
+        }
+    }
+    return false;
 }