Fix: config hot-reload watcher dropped immediately after spawn()

[aquinoonell]

Closes #113

Fix: move watcher into the background thread (let _watcher = watcher;)
so its lifetime matches the thread's, instead of ending the moment spawn()
returns.

Testing: confirmed via eprintln! instrumentation in reload_config and
pick_theme_step that reload_config was firing continuously at startup
with no user interaction, and that pick_theme_step's cur index was
always reading back 0. After the fix, reload_config only fires on an
actual edit to config.toml, and the theme picker preview steps correctly
through j/k.

@mikenikles

[aquinoonell]

I found that there's an open issue as well as a pull request fix: stop continuous config reloads - #101 who does the same thing as mines, although mines is more targeted. I found the bug after using the app.