feat: add download info pane to TUI

Splits the downloads area 60/40 with a new info pane that adapts to the
selected download's status: header (filename, user, size, status, save
path, bitrate, length) plus queued-specific fields (queue position, peer
slots, peer upload speed) or live progress/speed/ETA when transferring.

Plumbs queue position via a new ClientOperation::PlaceInQueueUpdate so
PlaceInQueueResponse messages from peers reach DownloadStore, and threads
search-time metadata (bitrate, length, peer slots/speed) onto Download
through a new download_with_metadata entry point.

soulseek-rs-lib/src/utils/mod.rs

@@ -2,6 +2,7 @@
 pub mod logger;
 pub mod lock;
 pub mod md5;
+pub mod path;
 pub mod thread_pool;
 pub mod zlib;
 

soulseek-rs-lib/src/utils/path.rs

@@ -0,0 +1,35 @@
+use std::path::PathBuf;
+
+pub fn expand_tilde(path: &str) -> PathBuf {
+    if let Some(stripped) = path.strip_prefix('~')
+        && let Ok(home) = std::env::var("HOME")
+    {
+        return PathBuf::from(home).join(stripped.trim_start_matches('/'));
+    }
+    PathBuf::from(path)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn no_tilde_returned_as_is() {
+        assert_eq!(expand_tilde("/abs/path"), PathBuf::from("/abs/path"));
+        assert_eq!(expand_tilde("rel/path"), PathBuf::from("rel/path"));
+    }
+
+    #[test]
+    fn tilde_expanded_when_home_set() {
+        // SAFETY: tests run single-threaded by default in this crate; if you
+        // ever run with --test-threads >1 this needs serialization.
+        unsafe {
+            std::env::set_var("HOME", "/home/test");
+        }
+        assert_eq!(expand_tilde("~"), PathBuf::from("/home/test"));
+        assert_eq!(
+            expand_tilde("~/Downloads"),
+            PathBuf::from("/home/test/Downloads")
+        );
+    }
+}