fix(mcp): add tokio-util for CancellationToken (T10.4)

http_transport.rs needs tokio_util::sync::CancellationToken directly;
rmcp pulls tokio-util transitively but that doesn't make it `use`-able
from our own crate without a direct Cargo.toml entry.

src-tauri/Cargo.toml

@@ -66,6 +66,7 @@ http-body-util = { version = "0.1", optional = true }
 http = { version = "1", optional = true }
 bytes = { version = "1", optional = true }
 tower-service = { version = "0.3", optional = true }
+tokio-util = { version = "0.7", optional = true }
 
 # audio / transcription / diarization / calendar are integrated per-phase and are
 # feature-gated so the CPU-only build always compiles (NFR-MNT-4).
@@ -127,6 +128,7 @@ sync = ["dep:keyring"]          # remote upload (WebDAV + OAuth providers)
 mcp = [
     "dep:rmcp", "dep:keyring", "dep:hyper", "dep:hyper-util",
     "dep:http-body-util", "dep:http", "dep:bytes", "dep:tower-service",
+    "dep:tokio-util",
 ]   # WhispAssist as an MCP server + hosted-AI creds
 
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