fix(tui): RAII TerminalGuard prevents terminal corruption on error exit (closes #8)

The previous code called std::process::exit(1) on file load failure,
bypassing all terminal restoration (disable_raw_mode, LeaveAlternateScreen,
show_cursor). This left the user's shell in a broken state with no echo
and no visible cursor.

Introduce TerminalGuard with Drop-based cleanup that fires on every exit
path: normal return, ? error propagation, and panic unwind. The guard
also handles partial initialization rollback (e.g. raw mode enabled but
alternate screen fails). Remove all process::exit calls from the guarded
scope.

crates/tui/src/main.rs

@@ -1,3 +1,6 @@
+use std::time::Duration;
+
+use anyhow::Context;
 use clap::Parser;
 
 mod app;
@@ -11,44 +14,93 @@ struct Cli {
     files: Vec<String>,
 }
 
+// ── RAII terminal guard ────────────────────────────────────────────
+//
+// Holds the ratatui Terminal and guarantees terminal state restoration
+// (disable raw mode, leave alternate screen, show cursor) on **every**
+// exit path: normal return, `?` error propagation, and panic unwind.
+//
+// IMPORTANT: `Drop` does **not** run on `std::process::exit` or
+// `panic = "abort"`.  Therefore we must never call `process::exit`
+// inside the guarded scope — use `?` to return `Err` instead.
+
+type Backend = ratatui::backend::CrosstermBackend<std::io::Stdout>;
+
+struct TerminalGuard {
+    terminal: ratatui::Terminal<Backend>,
+}
+
+impl TerminalGuard {
+    /// Enable raw mode, enter alternate screen, and create the terminal.
+    /// On partial failure, rolls back any steps that already succeeded.
+    fn enter() -> anyhow::Result<Self> {
+        crossterm::terminal::enable_raw_mode()
+            .map_err(|e| anyhow::anyhow!("enable_raw_mode: {e}"))?;
+
+        let mut stdout = std::io::stdout();
+        if let Err(e) = crossterm::execute!(stdout, crossterm::terminal::EnterAlternateScreen) {
+            let _ = crossterm::terminal::disable_raw_mode();
+            return Err(anyhow::anyhow!("EnterAlternateScreen: {e}"));
+        }
+
+        let backend = ratatui::backend::CrosstermBackend::new(stdout);
+        match ratatui::Terminal::new(backend) {
+            Ok(terminal) => Ok(Self { terminal }),
+            Err(e) => {
+                // Roll back: leave alternate screen + disable raw mode
+                let _ = crossterm::execute!(
+                    std::io::stdout(),
+                    crossterm::terminal::LeaveAlternateScreen
+                );
+                let _ = crossterm::terminal::disable_raw_mode();
+                Err(anyhow::anyhow!("Terminal::new: {e}"))
+            }
+        }
+    }
+
+    fn terminal(&mut self) -> &mut ratatui::Terminal<Backend> {
+        &mut self.terminal
+    }
+}
+
+impl Drop for TerminalGuard {
+    fn drop(&mut self) {
+        // Best-effort cleanup; suppress errors (Drop must not panic).
+        let _ = crossterm::terminal::disable_raw_mode();
+        let _ = crossterm::execute!(
+            self.terminal.backend_mut(),
+            crossterm::terminal::LeaveAlternateScreen
+        );
+        let _ = self.terminal.show_cursor();
+    }
+}
+
+// ── main ───────────────────────────────────────────────────────────
+
 fn main() -> anyhow::Result<()> {
     let cli = Cli::parse();
 
-    crossterm::terminal::enable_raw_mode()?;
-    let mut stdout = std::io::stdout();
-    crossterm::execute!(stdout, crossterm::terminal::EnterAlternateScreen)?;
-    let backend = ratatui::backend::CrosstermBackend::new(stdout);
-    let mut terminal = ratatui::Terminal::new(backend)?;
-
+    let mut guard = TerminalGuard::enter()?;
     let mut app = app::App::new();
     app.color_config = log_viewer_core::config::ColorConfig::load();
 
-    if let Some(file) = cli.files.first()
-        && let Err(e) = app.load_file(file)
-    {
-        eprintln!("Error loading file: {e}");
-        std::process::exit(1);
+    if let Some(file) = cli.files.first() {
+        app.load_file(file)
+            .with_context(|| format!("loading file {file}"))?;
     }
 
     while !app.should_quit {
         app.poll_background_indexer();
         app.poll_file_watcher();
-        terminal.draw(|frame| ui::render(frame, &mut app))?;
-        if crossterm::event::poll(std::time::Duration::from_millis(100))? {
+        guard.terminal().draw(|frame| ui::render(frame, &mut app))?;
+        if crossterm::event::poll(Duration::from_millis(100))? {
             match crossterm::event::read()? {
                 crossterm::event::Event::Key(key) => app.handle_key(key),
-                crossterm::event::Event::Resize(_w, _h) => {}
+                crossterm::event::Event::Resize(_, _) => {}
                 _ => {}
             }
         }
     }
 
-    crossterm::terminal::disable_raw_mode()?;
-    crossterm::execute!(
-        terminal.backend_mut(),
-        crossterm::terminal::LeaveAlternateScreen
-    )?;
-    terminal.show_cursor()?;
-
     Ok(())
 }