Prefix fire-and-forget calls (PubSub.broadcast, Oban enqueue, Logger,
update_*, etc.) with `_ =` so dialyzer knows the return value is
intentionally discarded. Wrap a few if/case expressions whose
branches produce mixed types the same way.
No behavior changes — only explicit acknowledgement of discarded
returns.
Warnings: 242 → 88.
wire time_format into scope, add settings UI selector, change defaults
to 24h, and replace all user-facing Calendar.strftime calls with
centralized TimeHelpers using the user's timezone and time format.
