feat(tui): results across the top, with panes you can hide, zoom and Tab through

The results pane now has the whole width, where a long file name needs it,
and Searches, Downloads and Info share the row underneath. Tab and Shift-Tab
walk the panes, z zooms the focused one to the window, w hides it so the
others widen, and its number brings it back. Every list takes the same
navigation keys, the shortcuts bar wraps instead of cutting keys off, and ?
lists every key by pane, scrolling on a short terminal.

Follows up on #55.

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

@@ -163,12 +163,15 @@ pub fn render_downloads_pane(
         ])
     }));
 
+    // Sized to the widest value each column shows in ordinary use — a
+    // three-digit megabyte count on both sides of the progress — so the file
+    // name keeps whatever width the pane has beyond that.
     let widths = [
-        ratatui::layout::Constraint::Length(8),
-        ratatui::layout::Constraint::Fill(2),
-        ratatui::layout::Constraint::Length(15),
-        ratatui::layout::Constraint::Length(25),
+        ratatui::layout::Constraint::Length(6),
+        ratatui::layout::Constraint::Fill(1),
         ratatui::layout::Constraint::Length(12),
+        ratatui::layout::Constraint::Length(25),
+        ratatui::layout::Constraint::Length(10),
     ];
 
     let table = Table::new(rows, widths)