Replace compile-time HMAC key with USB provisioning

ESP firmware:
- Store HMAC key in NVS via Preferences library instead of compile-time
  hmac_key.h
- KEY? responds with HMAC_KEY:<hex> (if provisioned) or NO_KEY
- New SET_KEY:<64-hex> command stores key in NVS and responds OK
- Boot messages updated to reflect provisioning state

Android app:
- UsbKeyReceiver detects NO_KEY response and auto-provisions:
  generates random 32-byte key via SecureRandom, sends SET_KEY,
  waits for OK, saves key to SharedPreferences
- Read loop now breaks on both HMAC_KEY: and NO_KEY responses

Cleanup:
- Removed tools/generate_hmac_key.py and src/hmac_key.h
- Removed hmac_key.h from .gitignore
- README updated for new provisioning flow

tools/generate_hmac_key.py

@@ -1,53 +0,0 @@
-#!/usr/bin/env python3
-"""Generate src/hmac_key.h with a random 32-byte HMAC-SHA256 key.
-
-Usage:
-    python3 tools/generate_hmac_key.py
-
-This creates src/hmac_key.h (gitignored) and prints the hex key to stdout.
-The hex key should be entered in the Android app's Settings screen.
-"""
-import os
-import sys
-import binascii
-
-KEY_LENGTH = 32
-OUTPUT_FILE = os.path.join("src", "hmac_key.h")
-
-
-def generate():
-    key = os.urandom(KEY_LENGTH)
-    hex_key = binascii.hexlify(key).decode().upper()
-
-    header = f"""#ifndef HMAC_KEY_H
-#define HMAC_KEY_H
-
-// AUTO-GENERATED by tools/generate_hmac_key.py
-// Do NOT commit this file. Do NOT edit by hand.
-// To regenerate: python3 tools/generate_hmac_key.py
-
-const uint8_t HMAC_KEY[{KEY_LENGTH}] = {{
-    {', '.join(f'0x{b:02x}' for b in key)}
-}};
-
-#endif // HMAC_KEY_H
-"""
-
-    os.makedirs("src", exist_ok=True)
-    with open(OUTPUT_FILE, "w") as f:
-        f.write(header)
-
-    print(f"HMAC key written to {OUTPUT_FILE}")
-    print(f"Enter this key in the Android app Settings:")
-    print(f"  {hex_key}")
-    print()
-    print(f"ESP serial output after boot will show the same key.")
-
-
-if __name__ == "__main__":
-    if os.path.exists(OUTPUT_FILE):
-        overwrite = input(f"{OUTPUT_FILE} already exists. Overwrite? [y/N] ")
-        if overwrite.lower() != 'y':
-            print("Aborted.")
-            sys.exit(0)
-    generate()