feat(ui): scrollable help menu, Esc-to-close, + blush/matrix/wraith themes

The help overlay now scrolls (↑/↓, PgUp/PgDn, Home/End) with a position
indicator and only Esc dismisses it, so stray keystrokes can't close a menu
that overflows the screen. Adds three bundled vestments (blush, matrix,
wraith); they're auto-discovered via Theme::available(), so they appear in
the menu and /theme list with no hardcoded entries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/src/app.rs

@@ -128,6 +128,8 @@ pub struct App {
     pub sbx_scroll: usize,
     /// Whether the help overlay is showing.
     pub show_help: bool,
+    /// Vertical scroll offset (rows) into the help overlay when it doesn't fit.
+    pub help_scroll: u16,
     /// A reconnect handshake is in flight (Ctrl-R after a disconnect).
     pub reconnecting: bool,
     /// Transient error shown as a popup over the clergy (cleared on next keypress).
@@ -159,6 +161,7 @@ impl App {
             chat_scroll: 0,
             sbx_scroll: 0,
             show_help: false,
+            help_scroll: 0,
             reconnecting: false,
             error: None,
             password: String::new(),
@@ -610,9 +613,29 @@ pub async fn run(params: net::ConnParams, mut session: Session, mut theme: Theme
                                 });
                             }
                         } else if app.show_help {
-                            app.show_help = false; // any key dismisses the overlay
+                            // While the help overlay is up, arrows / paging scroll it
+                            // (when it's taller than the screen); only Esc closes, so
+                            // stray keystrokes can't dismiss a menu you're still reading.
+                            let max = term
+                                .size()
+                                .map(|s| ui::help_max_scroll(s.width, s.height, &theme))
+                                .unwrap_or(0);
+                            match k.code {
+                                KeyCode::Up => app.help_scroll = app.help_scroll.saturating_sub(1),
+                                KeyCode::Down => app.help_scroll = (app.help_scroll + 1).min(max),
+                                KeyCode::PageUp => app.help_scroll = app.help_scroll.saturating_sub(10),
+                                KeyCode::PageDown => app.help_scroll = (app.help_scroll + 10).min(max),
+                                KeyCode::Home => app.help_scroll = 0,
+                                KeyCode::End => app.help_scroll = max,
+                                KeyCode::Esc => {
+                                    app.show_help = false; // Esc dismisses the overlay
+                                    app.help_scroll = 0;
+                                }
+                                _ => {} // ignore other keys so the menu stays put
+                            }
                         } else if k.code == KeyCode::F(1) {
                             app.show_help = true;  // F1 from any mode
+                            app.help_scroll = 0;
                         } else if k.code == KeyCode::F(2) {
                             if app.sandbox.is_none() {
                             } else if app.can_drive() {
@@ -876,6 +899,7 @@ fn handle_command(
     let room = &session.room;
     if line == "/help" || line == "/?" {
         app.show_help = true;
+        app.help_scroll = 0;
     } else if line == "/pw" || line == "/password" {
         // Show the room password locally (never broadcast). Handy when the
         // server's password was autogenerated and you need to read it off / share