feat(tui): navigate and confirm while filtering search results

In results filter mode, Up/Down move the selection over the filtered
list and Enter confirms the filter (keeps the query, returns to normal
results keys). Footer hints updated.

soulseek-rs/src/ui/main_tui/render.rs

@@ -372,6 +372,8 @@ impl MainTui {
                 FocusedPane::Results if self.state.results_is_filtering => {
                     vec![
                         ("Type", "filter"),
+                        ("↑/↓", "navigate"),
+                        ("Enter", "confirm"),
                         ("Esc", "clear filter"),
                         ("1-3", "focus pane"),
                         ("q", "quit"),