feat: use latest flet, try to fix threading issues

see also, previous comment in pyproject.toml (removed in this commit).

there is some event loop error happening on app exit with latest flet,
which may or may not be solved with this commit.  but since flet is
still essentially in beta phase, seems better to stay on latest
version instead of holding back.

this tries to fix the issue(s) in the following ways:

- timestamp thread is now "daemonized"
- if runtime error happens in timestamp control, ignore and exit thread
- if runtime error happens in main thread error handler, also ignore

we'll see what happens next after some usage

pyproject.toml

@@ -28,13 +28,8 @@ dependencies = [
         "rattail[db]",
         "typer",
 
-        # TODO: unfortunately the latest Flet versions as of writing
-        # (to 0.22.1) include a bug..when running `wuttapos open`
-        # command then quitting the app, you get an error "Event loop
-        # is closed" - i tried several versions of python (to 3.12.3)
-        # but nothing worked despite the claims made in github issue,
-        # https://github.com/flet-dev/flet/issues/2848
-        "flet<0.21",
+        # nb. for now we are just testing one version, for simplicity
+        "flet==0.23.2",
 ]