perf: UITheme::getMetrics const and const-ref usage (#1094)

## Summary

**What is the goal of this PR?**

Small cleanup to make getTheme and getMetrics methods on UITheme const.
They return const refs, so updated call sites to use `const auto&`.

Realistically this won't make much performance difference, but it better
conveys the nature of theme metrics being shared const state.

---

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src/activities/settings/OtaUpdateActivity.cpp

@@ -84,7 +84,7 @@ void OtaUpdateActivity::render(Activity::RenderLock&&) {
     return;
   }
 
-  auto metrics = UITheme::getInstance().getMetrics();
+  const auto& metrics = UITheme::getInstance().getMetrics();
   const auto pageWidth = renderer.getScreenWidth();
   const auto pageHeight = renderer.getScreenHeight();