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/rand_chacha-9523eb647dbe939b/invoked.timestamp

@@ -0,0 +1 @@
+This file has an mtime of when this was started.

warzone/target/debug/.fingerprint/rand_chacha-9523eb647dbe939b/lib-rand_chacha

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+82ac997b59e0b729

warzone/target/debug/.fingerprint/rand_chacha-9523eb647dbe939b/lib-rand_chacha.json

@@ -0,0 +1 @@
+{"rustc":13850170861107434965,"features":"[\"std\"]","declared_features":"[\"default\", \"serde\", \"serde1\", \"simd\", \"std\"]","target":15766068575093147603,"profile":5347358027863023418,"path":12916168183201262489,"deps":[[12919011715531272606,"ppv_lite86",false,3196748534240690378],[18130209639506977569,"rand_core",false,14392578981742011186]],"local":[{"CheckDepInfo":{"dep_info":"debug/.fingerprint/rand_chacha-9523eb647dbe939b/dep-lib-rand_chacha","checksum":false}}],"rustflags":[],"config":2069994364910194474,"compile_kind":0}