feat(cli)!: replace the one-shot commands with a scriptable surface

The one-shot half of the binary could not finish its headline task without a
human at the keyboard: `search` opened a full-screen picker, panicked with
exit 101 when there was no terminal, and no command could fetch a known file
at all. Everything exited 0 whatever happened, wrote emoji prose to stdout,
and ignored --download-dir, SOULSEEK_DOWNLOAD_DIR and config.toml.

The commands are now built for scripts and agents:

  search  download  get  browse  room list|say|listen  message send|read

stdout carries records only (tab-separated, or NDJSON under --json), stderr
carries progress, and the exit code carries the verdict: 2 usage, 3 connect,
4 no results, 5 timed out, 6 transfer failed. `get` does search, rank and
fetch in one step, and `search --json | jq ... | download --stdin` composes.
Global flags parse in any position and every one has a SOULSEEK_ variable and
a config.toml key resolved through a single precedence chain.

Defects found and fixed along the way, each covered by a test:

- a download with an unknown size was finalized against zero bytes and saved
  as an empty file; the size now comes from the peer's share listing
- transfers are keyed by a hash of the remote path, so the same path from two
  peers shadowed itself and two files sharing a basename overwrote each
  other; both collisions are now dropped with a warning
- `message send` and `room say` slept 500ms and claimed success; they now
  wait for a server reply queued behind the message before reporting
- Client::login waited on an unbounded channel and never returned when the
  server went quiet

Deleted the TUI picker the old `search` drove (download_selector,
MultiDownloadProgress, SearchConfig): 1,573 lines out, 926 in. The
interactive TUI is otherwise untouched.

BREAKING CHANGE: `rooms` and `chat` become `room list|say|listen` and
`message send|read`; `--disable-listener` becomes `--no-listener`; the
unprefixed DISABLE_LISTENER, LISTENER_PORT, LOG_FILE and
MAX_CONCURRENT_DOWNLOADS variables are now SOULSEEK_-prefixed; `search` no
longer opens a picker or downloads anything.

soulseek-rs/src/ui/mod.rs

@@ -1,4 +1,3 @@
-mod download_selector;
 mod downloads;
 pub mod login;
 mod main_tui;
@@ -6,8 +5,7 @@ mod panes;
 mod styles;
 mod utils;
 
-pub use download_selector::FileSelector;
-pub use downloads::{render_download_stats, show_multi_download_progress};
+pub use downloads::render_download_stats;
 pub use main_tui::launch_main_tui;
 pub use styles::*;
 pub use utils::*;