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/.gitignore b/.gitignore
index b2f1400..80cd5ed 100644
--- a/.gitignore
+++ b/.gitignore
@@ -19,4 +19,11 @@ tests/unit/test_identity
 tests/unit/test_nostr_event
 tests/unit/test_cashu
 tests/unit/test_session
+tests/unit/test_mcp_handler
+tests/unit/test_mint_health
+tests/unit/test_tollgate_client
+tests/unit/test_cvm_server
+tests/unit/test_lnurl_pay
+tests/unit/test_lightning_payout
+tests/unit/test_nip04
 interop/routers.env