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/mod.rs

@@ -7,7 +7,7 @@ mod styles;
 mod utils;
 
 pub use downloads::render_download_stats;
-pub use main_tui::launch_main_tui;
+pub use main_tui::{MainTui, launch_main_tui};
 pub use paging::*;
 pub use styles::*;
 pub use utils::*;