Fix type errors in watchdog thread cleanup

- `observer` can be None, if --restart-on-changes was not passed, but
  the hotkey F2 is pressed.
- `is_alive` is a bound method, not a property. It was definitely used
  like a property in the code I referenced.

I didn't fix these type checker errors for a long time because:
  1. they never caused actual problems for me, and
  2. when adding a type annotation `observer: Observer | None`, Pyright
     gets even _more_ confused, saying it's Unknown | None instead of
     BaseObserver | None. The secret trick was to leave it unannotated.
  3. Adding parentheses to make it `is_alive()` also made Pyright seem
     even more confused, saying it was Unknown — until I added the
     conditional.

src/textual_paint/paint.py

@@ -65,10 +65,11 @@ def restart_program():
 
     try:
         try:
-            observer.stop()
-            observer.join(timeout=1)
-            if observer.is_alive:
-                print("Timed out waiting for file change observer thread to stop.")
+            if observer:
+                observer.stop()
+                observer.join(timeout=1)
+                if observer.is_alive():
+                    print("Timed out waiting for file change observer thread to stop.")
         except RuntimeError as e:
             # Ignore "cannot join current thread" error
             # join() might be redundant, but I'm keeping it just in case something with threading changes in the future