fix: prevent UITheme memory leak on theme reload (#975)

## Summary

- `UITheme::currentTheme` was a raw owning pointer with no destructor,
  causing a heap leak every time `setTheme()` was called (e.g. on
  theme change via settings reload)

## Additional Context

- Replaced `const BaseTheme*` with `std::unique_ptr<BaseTheme>` so the
  previous theme object is automatically deleted on reassignment
- Added `<memory>` include to `UITheme.h`; allocations updated to
  `std::make_unique<>` in `UITheme.cpp`

---

### AI Usage

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Did you use AI tools to help write this code? _**NO**_ (identified by
claude though)

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/components/UITheme.h

@@ -1,12 +1,11 @@
 #pragma once
 
 #include <functional>
+#include <memory>
 
 #include "CrossPointSettings.h"
 #include "components/themes/BaseTheme.h"
 
-class MappedInputManager;
-
 class UITheme {
   // Static instance
   static UITheme instance;
@@ -26,7 +25,7 @@ class UITheme {
 
  private:
   const ThemeMetrics* currentMetrics;
-  const BaseTheme* currentTheme;
+  std::unique_ptr<BaseTheme> currentTheme;
 };
 
 // Helper macro to access current theme