feat: Add automatic workspace refresh with file watcher

Implement a polling-based file watcher that automatically refreshes
the file list when markdown files are created, deleted, or renamed.

- Add background thread that polls workspace every 2 seconds
- Watcher starts when workspace opens, stops when app exits
- Uses pure Rust std library (no external dependencies)
- Fixes issue where new folders like Act 2 don't appear until manual refresh

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/app/mod.rs

@@ -12,6 +12,8 @@ use eframe::egui;
 use std::collections::{HashMap, HashSet};
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
+use std::sync::mpsc;
+use std::thread;
 use std::time::{Duration, Instant};
 
 mod autocomplete;
@@ -51,6 +53,10 @@ type LtCheckMsg = (String, LtCheckResult);
 /// spelling matches found in it.
 type SpellCheckMsg = (String, Vec<crate::langtool::Match>);
 
+/// Message from the file watcher thread indicating the workspace needs a refresh.
+#[derive(Debug, Clone)]
+struct WatcherMsg;
+
 /// Which check currently owns the editor's underlines and the issues panel.
 #[derive(Clone, Copy, PartialEq, Eq)]
 enum IssueSource {
@@ -362,6 +368,10 @@ pub struct App {
     np_rx: Option<std::sync::mpsc::Receiver<ProjectMsg>>,
     /// Whether the new-project settings window is open.
     show_project_settings: bool,
+    /// Receiver for file change notifications from the workspace watcher.
+    watcher_rx: Option<mpsc::Receiver<WatcherMsg>>,
+    /// The file watcher thread handle, so it can be stopped when the workspace closes.
+    watcher_thread: Option<thread::JoinHandle<()>>,
 }
 
 impl App {
@@ -460,6 +470,8 @@ impl App {
             np_status: String::new(),
             np_rx: None,
             show_project_settings: false,
+            watcher_rx: None,
+            watcher_thread: None,
         };
         app.load_spell_dict();
         app.open_workspace();
@@ -496,6 +508,7 @@ impl eframe::App for App {
             self.refresh_find();
         }
 
+        self.poll_watcher();
         self.poll_lt();
         self.poll_settings_test();
         self.poll_spell();
@@ -610,6 +623,7 @@ impl eframe::App for App {
         self.save_current();
         self.persist_order();
         self.persist_titles();
+        self.stop_watcher();
         self.config.save();
     }
 }