feat(tui): run the highlighted search again with S

The search list survives a restart, but a restored query was only ever a
reminder: seeing what the network has now meant typing it out again. In
the Searches pane, `S` now re-runs the highlighted query in its place — no
duplicate row, no reordering — cancelling a run still collecting first and
starting the Results display over for what comes back.

The key is listed in the shortcut bar, the `?` overlay, the README and the
website. `start_search` is split into launching a search and showing it,
so both paths share one implementation.

A new `tui_e2e` suite drives the real window over a real session against
soulfind: keys in, a screen out, a sharing peer on the other end. It
restores a query from a previous run's state directory, presses `S`, and
checks the peer's file lands on screen — then again once the query has
been on the wire, proving a repeat is answered and the display does not
stack. To make that possible `MainTui` is exported with `handle_key_event`,
`render`, `poll_session` and a `state()` accessor, and CI runs the suite
with `SOULSEEK_E2E_REQUIRED=1`.

Closes #85

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

@@ -63,6 +63,7 @@ const HELP_RIGHT: &[(&str, &[(&str, &str)])] = &[
         "Searches",
         &[
             ("Enter", "show its results"),
+            ("S", "run the search again"),
             ("d", "remove the search"),
             ("C", "clear every search"),
             ("c", "chat rooms"),
@@ -126,7 +127,8 @@ fn filter_keys(
 }
 
 impl MainTui {
-    pub(super) fn render(&mut self, frame: &mut Frame) {
+    /// Draw the whole window.
+    pub fn render(&mut self, frame: &mut Frame) {
         let shortcuts = self.shortcut_rows(frame.area().width);
         let mut constraints = vec![
             Constraint::Length(3), // Status bar
@@ -631,6 +633,7 @@ impl MainTui {
             let mut keys = match self.state.focused_pane {
                 FocusedPane::Searches => vec![
                     ("s", "search"),
+                    ("S", "search again"),
                     ("Enter", "results"),
                     ("d", "remove"),
                     ("C", "clear all"),