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/components/UITheme.h

@@ -14,8 +14,8 @@ class UITheme {
   UITheme();
   static UITheme& getInstance() { return instance; }
 
-  const ThemeMetrics& getMetrics() { return *currentMetrics; }
-  const BaseTheme& getTheme() { return *currentTheme; }
+  const ThemeMetrics& getMetrics() const { return *currentMetrics; }
+  const BaseTheme& getTheme() const { return *currentTheme; }
   void reload();
   void setTheme(CrossPointSettings::UI_THEME type);
   static int getNumberOfItemsPerPage(const GfxRenderer& renderer, bool hasHeader, bool hasTabBar, bool hasButtonHints,