Replace HMAC-SHA256 with AES-256-GCM + ECDH key exchange for BLE relay commands (issue #2)

- New: src/ecdh_helper.h — ECDH keypair gen, shared secret, pubkey HMAC sign/verify
- Server (src/main.cpp): ECDH handshake callback, AES-GCM decrypt, PSK from NVS
- Client (src/main.cpp→client.cpp): ECDH after connect, AES-GCM encrypt, PSK from NVS
- Android: ECDH (KeyAgreement) + AES-GCM (Cipher) in BleRelayManager.kt
- PSK provisioned via USB CDC SET_KEY (SettingsActivity.kt)
- PrefsHelper: hmac_key → psk
- Bumped versions: server=18, client=4
- NVS namespace: hmac→syl, key:hmac_key→psk
- BLE ECDH characteristic: beb5483e-...-f26dc (WRITE_NR|NOTIFY)

docs/plan-issue-2-aes-ecdh.md

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+# Plan: Issue #2 — AES-256-GCM + ECDH + PSK voor BLE relay commands
+
+## Beveiligingsmodel
+
+### Provisioning (eenmalig, USB CDC)
+Android genereert 32B PSK (pre-shared key) en provisioned beide ESP's:
+- `SET_KEY` → Server ESP (NVS `syl/psk`)
+- `SET_KEY` → Client ESP (NVS `syl/psk`)
+
+### BLE ECDH handshake (elke sessie)
+```
+Client                                         Server
+  |── pubkey(65B) || HMAC(PSK,pubkey)(32B) ─────>|  verify HMAC met PSK
+  |                                               |  genereer eigen keypair
+  |<─ pubkey(65B) || HMAC(PSK,pubkey)(32B) ──────|  verify HMAC met PSK
+  |                                               |
+AES_KEY = SHA-256(ECDH_shared_secret)         AES_KEY = SHA-256(ECDH_shared_secret)
+```
+
+- PSK verlaat nooit het apparaat over BLE — alleen HMAC ervan
+- Zonder PSK kan MITM geen geldige HMAC voor fake pubkey produceren
+- Per-sessie AES keys → perfect forward secrecy
+- PSK gelekt? → her-provisionen via USB CDC
+
+## Packet structuren
+
+### ECDH handshake packet (97 bytes)
+```
+pubkey[65] || HMAC-SHA256(PSK, pubkey)[32]
+```
+BLE characteristic: `beb5483e-...-f26dc`, properties `WRITE_NR | NOTIFY`
+
+### Relay command (29 bytes, AES-256-GCM)
+```
+seq(4) || nonce(8) || ciphertext_cmd(1) || tag(16)
+```
+Nonce voor GCM = `seq(4) || random(8)` = 12 bytes.
+Sequence > lastSeq voor replay protection per sessie.
+
+## NVS / SharedPreferences
+
+| Opslag | Sleutel | Waarde |
+|---|---|---|
+| ESP NVS `syl` | `psk` | 32B PSK |
+| ESP NVS `syl` | `uuid` | 16B device UUID |
+| Android Prefs | `psk` | 64-char hex PSK |
+| Android Prefs | `device_uuid` | 32-char hex UUID |
+
+## Bestandswijzigingen
+
+### Nieuw
+- `src/ecdh_helper.h` — ECDH keypair generatie + shared secret (mbedtls/ecdh.h)
+
+### Gewijzigd
+- `src/main.cpp` — HMAC → ECDH handshake + AES-GCM decrypt; PSK uit NVS
+- `src/client.cpp` — HMAC → ECDH handshake + AES-GCM encrypt; PSK uit NVS
+- `android/.../BleRelayManager.kt` — HMAC → ECDH (KeyAgreement) + AES-GCM (Cipher)
+- `android/.../SettingsActivity.kt` — SET_KEY schrijft voortaan PSK
+- `android/.../PrefsHelper.kt` — `hmac_key` → `psk`
+- `version.txt` — bump server + client versies
+
+## Werkt NIET met oude firmware/app
+Geen backward compatibility: schone break. HMAC packets worden niet meer geaccepteerd.