perf(tui): build only the visible page of result rows

Scrolling a long results list was laggy: every frame built a table row
for every result, and ratatui then drew the forty-odd that fit. At the
10,000-file cap that cost 25-32ms a frame in release (69ms in debug), in
attached and standalone mode alike, so a held key stuttered.

The pane now slices to the page ratatui would show, by its own rule:
stay where the last frame left off, pulled along only as far as keeps
the selection in view. The table gets a page-local state and the page
start is written back into the real one. The page is counted with the
same page_of that PgUp/PgDn use, so paging and drawing agree, and a
selection past the end lands on the last row as ratatui put it. Idle
frames drop to 1.5ms and a keypress to 0.6ms at the same size, measured
live in both modes.

ratatui has no lazy rows of its own: the lazy_rows proposal was not
merged and moved to the experimental ratatui-table crate, so windowing
in the caller is the recommended approach today.

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

@@ -1,6 +1,6 @@
 use super::MainTui;
 use crate::models::{CommandBarMode, FocusedPane, LogView};
-use crate::ui::pane_block;
+use crate::ui::page_of;
 use crate::ui::panes::{
     InfoSubject, name_end_offset, query_end_offset, selected_transfer,
     transfer_name_end_offset, upload_display_name,
@@ -8,7 +8,7 @@ use crate::ui::panes::{
 use ratatui::crossterm::event::{
     KeyCode, KeyEvent, KeyModifiers, MouseButton, MouseEvent, MouseEventKind,
 };
-use ratatui::layout::{Position, Rect};
+use ratatui::layout::Position;
 use ratatui::widgets::TableState;
 
 const NAME_SCROLL_STEP: isize = 8;
@@ -575,16 +575,6 @@ impl MainTui {
     }
 }
 
-/// Rows a list in `area` shows at once, less the `chrome` rows above it: a
-/// table header, a popup's tab bar and compose line. At least one, so a page
-/// key always moves, even before the first draw.
-fn page_of(area: Option<Rect>, chrome: u16) -> usize {
-    area.map_or(0, |area| {
-        usize::from(pane_block(false).inner(area).height.saturating_sub(chrome))
-    })
-    .max(1)
-}
-
 /// A move over a whole list at once: to either end, or by a number of rows
 /// (negative is up).
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]