fix(terminal): switch to alternate screen buffer for clean exit

TUI frame was left in terminal after exit. Use EnterAlternateScreen/
LeaveAlternateScreen so exiting restores the original terminal content.

src/terminal.rs

@@ -1,22 +1,11 @@
 //! Terminal initialization, restoration, and panic hook for bbs-md.
 //!
-//! # Design
-//!
-//! This module uses the main screen buffer (no alternate screen) for the immersive BBS feel.
-//! On exit, TUI output remains visible in the terminal scroll history — the user can scroll
-//! up to review what they read, and the goodbye message persists after the process exits.
-//!
 //! # Broken Pipe Safety (LIFE-04)
 //!
 //! All write operations in the TUI must use `let _ =` or handle `ErrorKind::BrokenPipe`.
 //! SSH connections can close mid-write; Rust propagates this as `ErrorKind::BrokenPipe`.
-//! The actual enforcement happens in the event loop (Plan 03), but `restore_terminal()`
+//! The actual enforcement happens in the event loop, but `restore_terminal()`
 //! already follows this pattern with `let _ =` on every call.
-//!
-//! # Important
-//!
-//! Do NOT call `ratatui::restore()` — it enters alternate screen via `LeaveAlternateScreen`,
-//! which we never entered. Use `restore_terminal()` from this module instead.
 
 // Functions in this module are used in Plan 02 and 03; suppress Phase 1 dead code warnings.
 #![allow(dead_code)]
@@ -24,26 +13,24 @@
 use std::io::stdout;
 use ratatui::crossterm::{
     execute,
-    terminal::{enable_raw_mode, disable_raw_mode, Clear, ClearType},
-    cursor::{MoveTo, Show},
+    terminal::{enable_raw_mode, disable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
+    cursor::Show,
 };
 use ratatui::{backend::CrosstermBackend, Terminal, TerminalOptions, Viewport};
 
 /// Type alias for the terminal used throughout the application.
 pub type Term = Terminal<CrosstermBackend<std::io::Stdout>>;
 
-/// Initialize the terminal on the main screen buffer with raw mode enabled.
+/// Initialize the terminal with alternate screen buffer and raw mode.
 ///
-/// This does NOT enter the alternate screen buffer. Instead it clears the main screen
-/// so the TUI occupies the user's actual terminal. On exit, TUI output remains in
-/// scroll history.
+/// Uses the alternate screen so the TUI exits cleanly without leaving artifacts.
 ///
 /// # Errors
 ///
-/// Returns an error if raw mode cannot be enabled or if the screen cannot be cleared.
+/// Returns an error if raw mode cannot be enabled or the alternate screen cannot be entered.
 pub fn init_terminal() -> std::io::Result<Term> {
     enable_raw_mode()?;
-    execute!(stdout(), Clear(ClearType::All), MoveTo(0, 0))?;
+    execute!(stdout(), EnterAlternateScreen)?;
     let backend = CrosstermBackend::new(stdout());
     Terminal::with_options(backend, TerminalOptions {
         viewport: Viewport::Fullscreen,
@@ -52,13 +39,11 @@ pub fn init_terminal() -> std::io::Result<Term> {
 
 /// Restore the terminal to a usable state.
 ///
-/// Disables raw mode and shows the cursor. Every operation uses `let _ =` to suppress
-/// errors — this function is called from cleanup paths including the panic hook, where
-/// panicking would hide the original error.
-///
-/// Do NOT call `ratatui::restore()` — it unconditionally calls `LeaveAlternateScreen`,
-/// which corrupts the terminal since we never entered the alternate screen.
+/// Leaves alternate screen, disables raw mode, and shows the cursor. Every operation
+/// uses `let _ =` to suppress errors — this function is called from cleanup paths
+/// including the panic hook, where panicking would hide the original error.
 pub fn restore_terminal() {
+    let _ = execute!(std::io::stdout(), LeaveAlternateScreen);
     let _ = disable_raw_mode();
     let _ = execute!(std::io::stdout(), Show);
 }
@@ -75,6 +60,10 @@ pub fn install_panic_hook() {
     let original_hook = std::panic::take_hook();
     std::panic::set_hook(Box::new(move |panic_info| {
         // Restore terminal — use let _ to avoid double-panic if cleanup itself fails
+        let _ = ratatui::crossterm::execute!(
+            std::io::stdout(),
+            ratatui::crossterm::terminal::LeaveAlternateScreen
+        );
         let _ = ratatui::crossterm::terminal::disable_raw_mode();
         let _ = ratatui::crossterm::execute!(
             std::io::stdout(),