feat: add live font preview pane to font selection screen (#2349)

## Summary
* Adds a live font preview pane to the font selection screen so users
can see how a font looks before committing to it.
* Changes
* A preview pane occupying the top 30% of the font selection screen,
rendering sample pangram text in the previewed font
* A two-step confirm flow: first press (enter button) previews the font,
second press selects it
* Back restores the original font settings, so browsing has no side
effects
* Layout dimensions cached in `onEnter()` to avoid redundant
recalculation between `loop()` and `render()`

## Additional Context
* Preview sample text is hardcoded English; didn't want to use AI for
translation as I would not be able to verify the output in most
languages...
* The preview pane reduces visible list height; this is compensated by
passing the reserved height into `getNumberOfItemsPerPage`

---
### AI Usage
Did you use AI tools to help write this code? **PARTIALLY**
AI use for assisting in coding and in writing the PR description.

src/components/themes/BaseTheme.h

@@ -32,6 +32,9 @@ struct ThemeMetrics {
   int headerHeight;
   int verticalSpacing;
 
+  int previewPadding;
+  int previewHeightPercent;
+
   int contentSidePadding;
   int listRowHeight;
   int listWithSubtitleRowHeight;
@@ -111,6 +114,8 @@ constexpr ThemeMetrics values = {.batteryWidth = 15,
                                  .batteryBarHeight = 20,
                                  .headerHeight = 45,
                                  .verticalSpacing = 10,
+                                 .previewPadding = 12,
+                                 .previewHeightPercent = 30,
                                  .contentSidePadding = 20,
                                  .listRowHeight = 30,
                                  .listWithSubtitleRowHeight = 50,

src/components/themes/lyra/LyraTheme.h

@@ -12,6 +12,8 @@ constexpr ThemeMetrics values = {.batteryWidth = 16,
                                  .batteryBarHeight = 40,
                                  .headerHeight = 84,
                                  .verticalSpacing = 16,
+                                 .previewPadding = 12,
+                                 .previewHeightPercent = 30,
                                  .contentSidePadding = 20,
                                  .listRowHeight = 40,
                                  .listWithSubtitleRowHeight = 60,

src/components/themes/roundedraff/RoundedRaffTheme.h

@@ -11,6 +11,8 @@ constexpr ThemeMetrics values = {.batteryWidth = 15,
                                  .batteryBarHeight = 20,
                                  .headerHeight = 45,
                                  .verticalSpacing = 10,
+                                 .previewPadding = 12,
+                                 .previewHeightPercent = 30,
                                  .contentSidePadding = 20,
                                  .listRowHeight = 42,
                                  .listWithSubtitleRowHeight = 69,