fix: patch 3 file-watcher bugs blocking merge

1. Task leak: add watch::channel shutdown signal to WatcherRuntime so
   the debounce loop exits when stop_file_watcher drops the runtime.
   Adds tokio 'macros' feature for select!.

2. ID collision: use crypto.randomUUID() for synthetic diff message IDs
   instead of seconds-precision timestamps that collide on rapid writes.

3. Channel switch cleanup: call stopFileWatcher in the useEffect cleanup
   so navigating away stops the backend watcher. Also converts the
   pointless dynamic import to a static import.

desktop/src-tauri/Cargo.lock

@@ -6263,9 +6263,21 @@ dependencies = [
  "mio",
  "pin-project-lite",
  "socket2",
+ "tokio-macros",
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "tokio-macros"
+version = "2.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "385a6cb71ab9ab790c5fe8d67f1645e6c450a7ce006a33de03daa956cf70a496"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "tokio-rustls"
 version = "0.26.4"

desktop/src-tauri/Cargo.toml

@@ -35,7 +35,7 @@ tauri-plugin-updater = "2"
 tauri-plugin-process = "2"
 infer = "0.19"
 hex = "0.4"
-tokio = { version = "1", features = ["fs", "sync", "rt"] }
+tokio = { version = "1", features = ["fs", "sync", "rt", "macros"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 nostr = "0.37"

desktop/src-tauri/src/file_watcher.rs

@@ -43,6 +43,8 @@ pub struct ProjectDirConfig {
 /// Runtime state for an active file watcher.
 struct WatcherRuntime {
     _watcher: RecommendedWatcher,
+    /// Dropping the sender signals the debounce loop to exit.
+    _shutdown_tx: tokio::sync::watch::Sender<bool>,
 }
 
 /// App-level state that holds all active file watchers.
@@ -227,6 +229,9 @@ fn start_watcher(
     let project_dir_owned = project_dir.to_path_buf();
     let snapshots_ref = Arc::clone(&fw_state.snapshots);
 
+    // Shutdown signal: when the sender is dropped the loop exits.
+    let (shutdown_tx, mut shutdown_rx) = tokio::sync::watch::channel(false);
+
     // Debounce state: track last event time per file.
     let pending: Arc<Mutex<HashMap<PathBuf, Instant>>> = Arc::new(Mutex::new(HashMap::new()));
     let pending_clone = Arc::clone(&pending);
@@ -240,7 +245,11 @@ fn start_watcher(
     tauri::async_runtime::spawn(async move {
         let debounce_delay = Duration::from_millis(500);
         loop {
-            tokio::time::sleep(Duration::from_millis(250)).await;
+            // Exit when the WatcherRuntime (and its shutdown_tx) is dropped.
+            tokio::select! {
+                _ = shutdown_rx.changed() => break,
+                _ = tokio::time::sleep(Duration::from_millis(250)) => {}
+            }
 
             let ready_paths: Vec<PathBuf> = {
                 let mut pending_guard = match pending_clone.lock() {
@@ -369,6 +378,7 @@ fn start_watcher(
 
     Ok(WatcherRuntime {
         _watcher: watcher,
+        _shutdown_tx: shutdown_tx,
     })
 }
 

desktop/src/features/channels/ui/ChannelScreen.tsx

@@ -32,6 +32,11 @@ import { useLocalFileDiffs } from "@/features/messages/useLocalFileDiffs";
 import { PresenceBadge } from "@/features/presence/ui/PresenceBadge";
 import { useUsersBatchQuery } from "@/features/profile/hooks";
 import { mergeCurrentProfileIntoLookup } from "@/features/profile/lib/identity";
+import {
+  getProjectDir,
+  startFileWatcher,
+  stopFileWatcher,
+} from "@/shared/api/tauri";
 import type {
   Channel,
   Identity,
@@ -108,9 +113,6 @@ export function ChannelScreen({
     let cancelled = false;
     void (async () => {
       try {
-        const { getProjectDir, startFileWatcher } = await import(
-          "@/shared/api/tauri"
-        );
         const dir = await getProjectDir(activeChannelId);
         if (dir && !cancelled) {
           await startFileWatcher(activeChannelId);
@@ -121,6 +123,8 @@ export function ChannelScreen({
     })();
     return () => {
       cancelled = true;
+      // Stop the backend watcher so we don't leak one per channel visited.
+      void stopFileWatcher(activeChannelId).catch(() => {});
     };
   }, [activeChannelId]);
 

desktop/src/features/messages/useLocalFileDiffs.ts

@@ -52,7 +52,7 @@ export function useLocalFileDiffs(channelId: string | null) {
           return;
         }
 
-        const syntheticId = `local-diff-${payload.timestamp}-${payload.filePath}`;
+        const syntheticId = `local-diff-${crypto.randomUUID()}`;
 
         const message: TimelineMessage = {
           id: syntheticId,