fix: TUI selection underflow and filtered-results desync

Three related TUI correctness bugs:

- download_selector navigation (select_previous/next/last) computed
  items.len() - 1, panicking with an arithmetic underflow when the list was
  empty (e.g. a filter matched nothing). Index math is now in wrap-safe
  helpers that return no selection for an empty list.

- update_search_results overwrote the filtered results view with the full
  unfiltered list on every ~100ms poll, so an active filter silently stopped
  working as new results streamed in. It now re-derives the filtered view
  from the current query.

- The results pane rendered selection checkboxes by testing the display-row
  index against a selection set that stores unfiltered indices, so under an
  active filter the wrong rows showed as checked. It now maps the display
  index back through the filtered-index table.

Filtering and index-mapping logic extracted into pure, unit-tested helpers.

soulseek-rs/src/ui/panes/results_pane.rs

@@ -19,12 +19,33 @@ pub struct ResultsPaneParams<'a> {
     pub items: &'a [FileDisplayData],
     pub table_state: &'a mut TableState,
     pub selected_indices: &'a HashSet<usize>,
+    /// Maps a rendered row index to its index in the unfiltered results list.
+    /// `None` means the rendered rows are the unfiltered list (identity map).
+    pub original_indices: Option<&'a [usize]>,
     pub filter_query: &'a str,
     pub is_filtering: bool,
     pub focused: bool,
     pub active_search_query: Option<&'a str>,
 }
 
+/// Whether the rendered row `display_idx` is selected. `selected_indices` holds
+/// indices into the *unfiltered* results, so under an active filter the display
+/// index must be translated through `original_indices` first.
+fn row_is_selected(
+    display_idx: usize,
+    original_indices: Option<&[usize]>,
+    selected_indices: &HashSet<usize>,
+) -> bool {
+    let original = match original_indices {
+        Some(map) => match map.get(display_idx) {
+            Some(&o) => o,
+            None => return false,
+        },
+        None => display_idx,
+    };
+    selected_indices.contains(&original)
+}
+
 pub fn render_results_pane(
     frame: &mut Frame,
     area: Rect,
@@ -34,6 +55,7 @@ pub fn render_results_pane(
         items,
         table_state,
         selected_indices,
+        original_indices,
         filter_query,
         is_filtering,
         focused,
@@ -83,11 +105,12 @@ No results. Select a search from the Searches pane [1]. Or start new search [s
         .iter()
         .enumerate()
         .map(|(idx, file)| {
-            let checkbox = if selected_indices.contains(&idx) {
-                "[✓]"
-            } else {
-                "[ ]"
-            };
+            let checkbox =
+                if row_is_selected(idx, original_indices, selected_indices) {
+                    "[✓]"
+                } else {
+                    "[ ]"
+                };
 
             let bitrate_str = file
                 .bitrate
@@ -147,3 +170,34 @@ No results. Select a search from the Searches pane [1]. Or start new search [s
 
     frame.render_stateful_widget(table, area, table_state);
 }
+
+#[cfg(test)]
+mod tests {
+    use super::row_is_selected;
+    use std::collections::HashSet;
+
+    #[test]
+    fn identity_mapping_used_when_no_filter() {
+        let selected: HashSet<usize> = [1].into_iter().collect();
+        assert!(row_is_selected(1, None, &selected));
+        assert!(!row_is_selected(0, None, &selected));
+    }
+
+    #[test]
+    fn filtered_rows_resolve_through_original_indices() {
+        // Filter shows original rows 2, 5, 7 as display rows 0, 1, 2.
+        // The user selected original row 5.
+        let original_indices = [2usize, 5, 7];
+        let selected: HashSet<usize> = [5].into_iter().collect();
+        assert!(!row_is_selected(0, Some(&original_indices), &selected));
+        assert!(row_is_selected(1, Some(&original_indices), &selected));
+        assert!(!row_is_selected(2, Some(&original_indices), &selected));
+    }
+
+    #[test]
+    fn out_of_range_display_index_is_not_selected() {
+        let original_indices = [2usize, 5];
+        let selected: HashSet<usize> = [2, 5].into_iter().collect();
+        assert!(!row_is_selected(9, Some(&original_indices), &selected));
+    }
+}