fix: two medium bugs from the final adversarial review

- Retry was broken for the common case: retry_selected_download removed
  the failed download only from the UI list, not the client's
  DownloadStore. Since Client::download re-derives the same md5 token
  from the unchanged filename, retrying a still-Failed download left a
  duplicate-token entry; the token-migration path then deleted both,
  misrouting completion so the retried file downloaded to disk but the
  UI showed 'Queued' forever. Add Client::remove_download /
  DownloadStore::remove_by_file (status-agnostic) and clear the stale
  entry before re-issuing. Regression-tested.

- Chat-room member selection could point out of range: RoomEvent::UserLeft
  shrank the active room's member list without clamping user_selected, so
  the highlight silently shifted to the wrong user (or vanished, making
  'b'/'m' no-ops, and blanking the member pane). Clamp user_selected after
  every applied room event. Regression-tested.

soulseek-rs-lib/src/download_store.rs

@@ -84,6 +84,17 @@ impl DownloadStore {
         true
     }
 
+    /// Remove every download matching `username`/`filename` regardless of
+    /// status. Used before retrying a failed download so the stale entry (whose
+    /// md5-derived token collides with the retry's) can't shadow the fresh one.
+    /// Returns whether anything was removed.
+    pub fn remove_by_file(&mut self, username: &str, filename: &str) -> bool {
+        let before = self.downloads.len();
+        self.downloads
+            .retain(|d| !(d.username == username && d.filename == filename));
+        self.downloads.len() != before
+    }
+
     pub fn pause_by_file(&mut self, username: &str, filename: &str) -> bool {
         let Some(download) = self.get_by_file_mut(username, filename) else {
             return false;
@@ -272,6 +283,28 @@ mod tests {
         assert!(store.get_by_token(456).is_some());
     }
 
+    #[test]
+    fn remove_by_file_removes_regardless_of_status() {
+        let mut store = DownloadStore::new();
+        // A failed download (the retry case) plus a same-name duplicate that a
+        // token-migration could have left behind — both must go.
+        let mut failed = make_download(1, DownloadStatus::Failed(None));
+        failed.filename = "song.mp3".to_string();
+        store.add(failed);
+        let mut dup = make_download(2, DownloadStatus::Queued);
+        dup.filename = "song.mp3".to_string();
+        store.add(dup);
+        let mut other = make_download(3, DownloadStatus::Queued);
+        other.filename = "other.mp3".to_string();
+        store.add(other);
+
+        assert!(store.remove_by_file("peer", "song.mp3"));
+        assert!(store.get_by_token(1).is_none());
+        assert!(store.get_by_token(2).is_none());
+        assert!(store.get_by_token(3).is_some(), "other file untouched");
+        assert!(!store.remove_by_file("peer", "song.mp3"), "idempotent");
+    }
+
     #[test]
     fn collect_failed_tokens_notifies_and_lists_matching() {
         let mut store = DownloadStore::new();