Add Watch Folders settings page with full CRUD

- New settings page: Watch Folders with add/remove/edit controls
- Each watch folder has: path, linked preset dropdown, recursive toggle,
  active/inactive switch, and remove button
- Watch folder config persisted in AppConfig
- Empty state message when no folders configured

pixstrip-core/src/config.rs

@@ -1,5 +1,7 @@
 use serde::{Deserialize, Serialize};
 
+use crate::watcher::WatchFolder;
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(default)]
 pub struct AppConfig {
@@ -20,6 +22,7 @@ pub struct AppConfig {
     pub reduced_motion: bool,
     pub history_max_entries: usize,
     pub history_max_days: u32,
+    pub watch_folders: Vec<WatchFolder>,
 }
 
 impl Default for AppConfig {
@@ -42,6 +45,7 @@ impl Default for AppConfig {
             reduced_motion: false,
             history_max_entries: 50,
             history_max_days: 30,
+            watch_folders: Vec::new(),
         }
     }
 }