feat: config.toml settings layer (CLI > env > file > defaults)

New persist module in the binary crate (lib stays zero-dep): FileConfig
loaded from a platform-appropriate config path via the directories crate,
with resolution precedence CLI/env (clap) > config.toml > built-in
defaults. Clap defaults move into the resolver so the file layer can sit
between.

soulseek-rs/Cargo.toml

@@ -24,3 +24,10 @@ ratatui = "0.30.2"
 color-eyre = "0.6.5"
 dotenv = "0.15.0"
 igd-next = "0.17"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+toml = "0.9"
+directories = "6"
+
+[dev-dependencies]
+tempfile = "3"