ui: buttons: fix clippy warning about confusing lifetimes

[hnez]

This feels like something where clippy suggested the exact opposite in the past, but I am not in the mood to argue.

Fixes the following warnings:

 error: hiding a lifetime that's elided elsewhere is confusing
  --> src/ui/buttons.rs:41:29
   |
41 |         pub fn fetch_events(&mut self) -> Result<FetchEventsSynced, ()> {
   |                             ^^^^^^^^^            ----------------- the same lifetime is hidden here
   |                             |
   |                             the lifetime is elided here
   |
   = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
   = note: `-D mismatched-lifetime-syntaxes` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(mismatched_lifetime_syntaxes)]`
help: use `'_` for type paths
   |
41 |         pub fn fetch_events(&mut self) -> Result<FetchEventsSynced<'_>, ()> {
   |                                                                   ++++

This fixes the failed scheduled CI runs.