Phase 3: on-device Cashu wallet with mbedTLS secp256k1 + SPIFFS persistence + PSRAM

- wallet.c/h: secp256k1 ECP primitives (hash_to_curve, scalar_mul, point_add)
- wallet_persist.c/h: SPIFFS persistence with threshold-based write protection
- Fee accounting for swap (input_fee_ppk from /v1/keysets)
- Keyset fetch via /v1/keysets (586 bytes vs 21KB for /v1/keys)
- Wallet API: GET /wallet, POST /wallet/swap, POST /wallet/send
- Payment proofs auto-stored to wallet + persisted on SPIFFS
- PSRAM enabled for large allocations (ESP32-S3 has 8MB)
- Wallet init deferred to dedicated task (avoids sys_evt stack overflow)
- Cashu proof ID buffer size fixed (66 hex chars, not 16)
- HTTP client: added fetch_headers() call for proper response handling
- persist_threshold_sats config parameter (default: 1 sat)

1 files changed, 53 insertions, 0 deletions

diff --git a/main/wallet.h b/main/wallet.h
new file mode 100644
index 0000000..5089f93
--- /dev/null
+++ b/main/wallet.h
@@ -0,0 +1,53 @@
+#ifndef WALLET_H
+#define WALLET_H
+
+#include "esp_err.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+#define WALLET_MAX_PROOFS 50
+#define WALLET_MAX_KEYSETS 5
+#define WALLET_KEYSET_ID_LEN 68
+#define WALLET_SECRET_LEN 65
+#define WALLET_SIG_LEN 67
+
+typedef struct {
+    uint64_t amount;
+    char id[WALLET_KEYSET_ID_LEN];
+    char secret[WALLET_SECRET_LEN];
+    char c[WALLET_SIG_LEN];
+} wallet_proof_t;
+
+typedef struct {
+    char id[WALLET_KEYSET_ID_LEN];
+    char public_key_33[67];
+    uint64_t amount;
+    int input_fee_ppk;
+} wallet_keyset_t;
+
+typedef struct {
+    wallet_proof_t proofs[WALLET_MAX_PROOFS];
+    int proof_count;
+    wallet_keyset_t keysets[WALLET_MAX_KEYSETS];
+    int keyset_count;
+    uint64_t balance;
+} wallet_t;
+
+esp_err_t wallet_init(void);
+wallet_t *wallet_get(void);
+uint64_t wallet_balance(void);
+
+esp_err_t wallet_add_proofs(const wallet_proof_t *proofs, int count);
+esp_err_t wallet_remove_proof(int index);
+void wallet_clear(void);
+
+esp_err_t wallet_fetch_keysets(const char *mint_url);
+esp_err_t wallet_swap_proofs(const char *mint_url, int start_index, int count);
+
+esp_err_t wallet_create_token(char *out, size_t out_size, uint64_t amount,
+                               const char *mint_url);
+esp_err_t wallet_send(const char *mint_url, uint64_t amount,
+                       char *token_out, size_t token_out_size);
+
+void wallet_print_status(void);
+#endif