Gateway tray: dead_code warnings in tray/logic.rs on macOS

[gregjkal]

Part of #55.

cargo build --release -p gateway on macOS (both aarch64-apple-darwin and x86_64-apple-darwin, master 3121cdd) emits two dead_code warnings:

warning: function `grayed` is never used
   --> crates/gateway/src/tray/logic.rs:428:15

warning: function `error_tint` is never used
   --> crates/gateway/src/tray/logic.rs:438:15

Both are pub(crate) helpers in the tray icon logic that no macOS code path calls. CI runs clippy -D warnings, so either the macOS target is not covered by that job or these are gated in a way the job does not hit; a macOS clippy run fails today.

Done means

• Either the two functions are used on macOS, removed, or cfg-gated to the platforms that call them.
• cargo clippy -p gateway --target aarch64-apple-darwin -- -D warnings passes.