feat: install tab completion for bash, zsh, and fish (#16)

`completions install` generates the script from the same clap definition
the binary parses with, so it cannot drift from `--help`, and writes it
where each shell already looks. fish scans its own completions directory
unprompted; bash and zsh get one marked `source` line appended to their rc
file, because nothing puts a per-user directory on their search path by
default. `completions uninstall` takes back exactly those lines and leaves
the rest of the file alone.

soulseek-rs/Cargo.toml

@@ -28,6 +28,7 @@ workspace = true
 soulseek-rs-lib = { version = "8.0.0", path = "../soulseek-rs-lib" }
 
 clap = { version = "4.6.2", features = ["derive", "color", "wrap_help", "env"] }
+clap_complete = "4.6.7"
 ratatui = "0.30.2"
 color-eyre = "0.6.5"
 dotenv = "0.15.0"