Add file watcher for watch folder functionality

inotify-based folder watcher using the notify crate that detects new
image files, ignores non-image files, and supports start/stop lifecycle.
WatchFolder config struct for preset-linked watched directories.

pixstrip-core/tests/watcher_tests.rs

@@ -0,0 +1,115 @@
+use pixstrip_core::watcher::*;
+use std::sync::mpsc;
+
+#[test]
+fn watch_folder_serialization() {
+    let folder = WatchFolder {
+        path: "/home/user/photos".into(),
+        preset_name: "Blog Photos".into(),
+        recursive: true,
+        active: true,
+    };
+
+    let json = serde_json::to_string(&folder).unwrap();
+    let deserialized: WatchFolder = serde_json::from_str(&json).unwrap();
+    assert_eq!(deserialized.preset_name, "Blog Photos");
+    assert!(deserialized.recursive);
+}
+
+#[test]
+fn watcher_starts_and_stops() {
+    let dir = tempfile::tempdir().unwrap();
+    let folder = WatchFolder {
+        path: dir.path().to_path_buf(),
+        preset_name: "Test".into(),
+        recursive: false,
+        active: true,
+    };
+
+    let watcher = FolderWatcher::new();
+    let (tx, _rx) = mpsc::channel();
+
+    watcher.start(&folder, tx).unwrap();
+    assert!(watcher.is_running());
+
+    watcher.stop();
+    // Give the thread time to see the stop signal
+    std::thread::sleep(std::time::Duration::from_millis(600));
+    assert!(!watcher.is_running());
+}
+
+#[test]
+fn watcher_detects_new_image() {
+    let dir = tempfile::tempdir().unwrap();
+    let folder = WatchFolder {
+        path: dir.path().to_path_buf(),
+        preset_name: "Test".into(),
+        recursive: false,
+        active: true,
+    };
+
+    let watcher = FolderWatcher::new();
+    let (tx, rx) = mpsc::channel();
+
+    watcher.start(&folder, tx).unwrap();
+
+    // Wait for watcher to be ready
+    std::thread::sleep(std::time::Duration::from_millis(200));
+
+    // Create an image file
+    let img_path = dir.path().join("new_photo.jpg");
+    std::fs::write(&img_path, b"fake jpeg data").unwrap();
+
+    // Wait and check for event
+    let event = rx.recv_timeout(std::time::Duration::from_secs(3));
+    watcher.stop();
+
+    match event {
+        Ok(WatchEvent::NewImage(path)) => {
+            assert_eq!(path, img_path);
+        }
+        Ok(WatchEvent::Error(e)) => panic!("Unexpected error: {}", e),
+        Err(_) => panic!("No event received within timeout"),
+    }
+}
+
+#[test]
+fn watcher_ignores_non_image_files() {
+    let dir = tempfile::tempdir().unwrap();
+    let folder = WatchFolder {
+        path: dir.path().to_path_buf(),
+        preset_name: "Test".into(),
+        recursive: false,
+        active: true,
+    };
+
+    let watcher = FolderWatcher::new();
+    let (tx, rx) = mpsc::channel();
+
+    watcher.start(&folder, tx).unwrap();
+    std::thread::sleep(std::time::Duration::from_millis(200));
+
+    // Create a non-image file
+    std::fs::write(dir.path().join("readme.txt"), b"text file").unwrap();
+
+    // Should not receive any event
+    let result = rx.recv_timeout(std::time::Duration::from_secs(1));
+    watcher.stop();
+
+    assert!(result.is_err(), "Should not receive event for non-image file");
+}
+
+#[test]
+fn watcher_rejects_nonexistent_path() {
+    let folder = WatchFolder {
+        path: "/nonexistent/path/to/watch".into(),
+        preset_name: "Test".into(),
+        recursive: false,
+        active: true,
+    };
+
+    let watcher = FolderWatcher::new();
+    let (tx, _rx) = mpsc::channel();
+
+    assert!(watcher.start(&folder, tx).is_err());
+}