feat(tui): hierarchical browse tree with file/folder downloads

Wire browsing a user's shared files into the TUI as a modal tree popup:
- press 'b' on a highlighted search result to browse that result's owner, or
  'b' elsewhere to type a username (new CommandBarMode::Browse);
- a new models/browse.rs synthesizes the flat SharedDirectory list into a
  nested, collapsible tree (intermediate folders reconstructed from the
  backslash paths; file download paths kept byte-exact from the original names),
  with expand/collapse state and a flatten-to-rows renderer — fully unit-tested;
- the popup polls take_browse_result each tick (with a timeout), navigates with
  arrows/hjkl, expands/collapses folders, and queues downloads: Enter on a file,
  'd' on a file or an entire folder subtree (recursively) — all funneled into
  the existing Downloads pane;
- shortcuts bar advertises browse keys and surfaces our own sharing status.

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

@@ -0,0 +1,93 @@
+use crate::models::{BrowseState, BrowseStatus};
+use crate::ui::{
+    HIGHLIGHT_SYMBOL, border_style, border_type, error_style, format_bytes,
+    get_spinner_char, highlight_style, primary_style,
+};
+use ratatui::{
+    Frame,
+    layout::{Constraint, Rect},
+    text::Line,
+    widgets::{
+        Block, Borders, Cell, HighlightSpacing, Paragraph, Row, Table,
+        TableState,
+    },
+};
+
+/// Render the browse popup: a collapsible tree of a peer's shared files.
+pub fn render_browse_pane(
+    frame: &mut Frame,
+    area: Rect,
+    browse: &BrowseState,
+    table_state: &mut TableState,
+    spinner_state: usize,
+) {
+    let title = match browse.status {
+        BrowseStatus::Loaded => format!(
+            " Browse {} — {} files, {} folders  (Enter/d: download, Esc: close) ",
+            browse.username, browse.file_count, browse.folder_count
+        ),
+        _ => format!(" Browse {} ", browse.username),
+    };
+    let block = Block::default()
+        .borders(Borders::ALL)
+        .border_style(border_style(true))
+        .border_type(border_type(true))
+        .title(title);
+
+    match browse.status {
+        BrowseStatus::Loading => {
+            let text = format!(
+                "{} Requesting shared files from {}…",
+                get_spinner_char(spinner_state),
+                browse.username
+            );
+            frame.render_widget(Paragraph::new(text).block(block), area);
+        }
+        BrowseStatus::Empty => {
+            let text = format!("{} is not sharing any files.", browse.username);
+            frame.render_widget(Paragraph::new(text).block(block), area);
+        }
+        BrowseStatus::TimedOut => {
+            let text = Line::styled(
+                "Timed out waiting for the file list. Press b to retry.",
+                error_style(),
+            );
+            frame.render_widget(Paragraph::new(text).block(block), area);
+        }
+        BrowseStatus::Loaded => {
+            let rows: Vec<Row> = browse
+                .rows()
+                .iter()
+                .map(|row| {
+                    let indent = "  ".repeat(row.depth);
+                    let (label, size) = if row.is_folder {
+                        let glyph = if row.expanded { "▾" } else { "▸" };
+                        (
+                            Cell::from(format!("{indent}{glyph} {}", row.name))
+                                .style(primary_style()),
+                            Cell::from(String::new()),
+                        )
+                    } else {
+                        (
+                            Cell::from(format!("{indent}  {}", row.name)),
+                            Cell::from(
+                                row.size.map(format_bytes).unwrap_or_default(),
+                            ),
+                        )
+                    };
+                    Row::new(vec![label, size])
+                })
+                .collect();
+
+            let table =
+                Table::new(rows, [Constraint::Fill(1), Constraint::Length(12)])
+                    .row_highlight_style(highlight_style())
+                    .highlight_symbol(HIGHLIGHT_SYMBOL)
+                    .highlight_spacing(HighlightSpacing::Always)
+                    .block(block);
+
+            table_state.select(Some(browse.selected_row));
+            frame.render_stateful_widget(table, area, table_state);
+        }
+    }
+}

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

@@ -1,8 +1,10 @@
+mod browse_pane;
 mod download_info_pane;
 mod downloads_pane;
 mod results_pane;
 mod searches_pane;
 
+pub use browse_pane::render_browse_pane;
 pub use download_info_pane::render_download_info_pane;
 pub use downloads_pane::render_downloads_pane;
 pub use results_pane::{ResultsPaneParams, render_results_pane};