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.

lib/I18n/translations/czech.yaml

@@ -73,6 +73,7 @@ STR_LONG_PRESS_BEHAVIOR: "Chování při dlouhém stisknutí tlačítka"
 STR_LONG_PRESS_BEHAVIOR_OFF: "VYP"
 STR_LONG_PRESS_BEHAVIOR_SKIP: "Přeskočení kapitoly"
 STR_LONG_PRESS_BEHAVIOR_ORIENTATION: "Změna orientace"
+STR_FONT_PREVIEW_TEXT: "Příliš žluťoučký kůň úpěl ďábelské ódy"
 STR_FONT_FAMILY: "Rodina písem čtečky"
 STR_FONT_SIZE: "Velikost písma rozhraní"
 STR_LINE_SPACING: "Řádkování čtečky"