Fix X3DH + add web client served by warzone-server

X3DH fix:
- Added identity_encryption_key (X25519) to PreKeyBundle
- initiate() and respond() now use correct DH operations per Signal spec:
  DH1=IK_a*SPK_b, DH2=EK_a*IK_b, DH3=EK_a*SPK_b, DH4=EK_a*OPK_b
- All 17 tests pass including x3dh_shared_secret_matches

Web client (served at /):
- Identity generation with seed (stored in localStorage)
- Recovery from hex-encoded seed
- Auto-load saved identity on page load
- Fingerprint display (same format as CLI: xxxx:xxxx:xxxx:xxxx)
- Key registration with server via /v1/keys/register
- Chat UI with message polling (5s interval)
- Commands: /help, /info, /seed
- Dark theme matching warzone aesthetic

Both clients (CLI + Web) now exist:
- CLI: warzone init, warzone info, warzone recover
- Web: http://localhost:7700/ (served by warzone-server)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/target/debug/.fingerprint/chacha20poly1305-164db92f0257a983/lib-chacha20poly1305.json

@@ -0,0 +1 @@
+{"rustc":13850170861107434965,"features":"[\"alloc\", \"default\", \"getrandom\", \"rand_core\"]","declared_features":"[\"alloc\", \"default\", \"getrandom\", \"heapless\", \"rand_core\", \"reduced-round\", \"std\", \"stream\"]","target":2570101318813280072,"profile":5347358027863023418,"path":5317624892339363080,"deps":[[6192938164125971281,"poly1305",false,9143502308472071639],[7916416211798676886,"cipher",false,12705040680522176484],[11163181423074495534,"chacha20",false,7314229268116911351],[12865141776541797048,"zeroize",false,248298559044779970],[17797166225172937111,"aead",false,9117197295274567822]],"local":[{"CheckDepInfo":{"dep_info":"debug/.fingerprint/chacha20poly1305-164db92f0257a983/dep-lib-chacha20poly1305","checksum":false}}],"rustflags":[],"config":2069994364910194474,"compile_kind":0}