perf(crypto): native Argon2id via dryoc — 6x faster vault unlock

Phase A of the crypto/livekit rust-native migration.

Rust side
- dryoc crate (pure-rust libsodium-compat, no C toolchain)
- Tauri commands: crypto_random_bytes, crypto_secretbox_encrypt/decrypt,
  crypto_box_keypair, crypto_box_encrypt/decrypt, crypto_box_seal/open,
  crypto_pwhash — all bit-compatible with libsodium-wrappers-sumo
- Commands registered via invoke_handler in lib.rs
- All IPC payloads base64-encoded to survive serde_json

JS side
- lib/nativeCryptoOps.ts exposes pwhashArgon2id + randomBytesAsync
  plus optional secretbox accelerators for future call-site migration
- Native-first, WASM fallback on error or when VITE_USE_NATIVE_CRYPTO is
  false / in browser preview
- Argon2id call-sites migrated: secureFileStore.deriveKey and
  deviceBackup.deriveKey (covers vault unlock + backup/recovery flows)

Impact
- Vault unlock: ~1200ms → ~200ms (measured locally, Argon2id moderate)
- Per-message AEAD left on WASM-worker path: IPC overhead ~40µs would
  dominate any native speedup below ~100µs/op
- WASM stays installed as graceful fallback so browser-preview builds
  keep working and a native failure self-heals at runtime

apps/desktop/src-tauri/Cargo.toml

@@ -22,6 +22,12 @@ tauri-plugin-fs = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 
+# Pure-rust libsodium-compatible primitives. No C toolchain required so
+# cross-compile for mobile stays clean. API output is bit-compatible with
+# libsodium-wrappers-sumo for the ops we use (secretbox, box, pwhash).
+dryoc = { version = "0.7", default-features = false, features = ["serde"] }
+base64 = "0.22"
+
 [target."cfg(not(any(target_os = \"android\", target_os = \"ios\")))".dependencies]
 tauri-plugin-global-shortcut = "2"
 tauri-plugin-updater = "2"