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/uuid-8eacbf95b6de1aca/lib-uuid.json

@@ -0,0 +1 @@
+{"rustc":13850170861107434965,"features":"[\"default\", \"rng\", \"serde\", \"std\", \"v4\"]","declared_features":"[\"arbitrary\", \"atomic\", \"borsh\", \"bytemuck\", \"default\", \"fast-rng\", \"js\", \"macro-diagnostics\", \"md5\", \"rng\", \"rng-getrandom\", \"rng-rand\", \"serde\", \"sha1\", \"slog\", \"std\", \"uuid-rng-internal-lib\", \"v1\", \"v3\", \"v4\", \"v5\", \"v6\", \"v7\", \"v8\", \"zerocopy\"]","target":2422778461497348360,"profile":6224605917853037374,"path":14720306295300140488,"deps":[[6509165896255665847,"getrandom",false,14168734980082658649],[11899261697793765154,"serde_core",false,8090025122213632381]],"local":[{"CheckDepInfo":{"dep_info":"debug/.fingerprint/uuid-8eacbf95b6de1aca/dep-lib-uuid","checksum":false}}],"rustflags":[],"config":2069994364910194474,"compile_kind":0}