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/valencian.yaml

@@ -78,6 +78,7 @@ STR_LONG_PRESS_BEHAVIOR: "Comportament de prémer llargament el botó"
 STR_LONG_PRESS_BEHAVIOR_OFF: "Desactivat"
 STR_LONG_PRESS_BEHAVIOR_SKIP: "Saltar capítols"
 STR_LONG_PRESS_BEHAVIOR_ORIENTATION: "Canvi d'orientació"
+STR_FONT_PREVIEW_TEXT: "Jove xef, porti whisky amb quinze glaçons d'hidrogen, coi!"
 STR_FONT_FAMILY: "Família de fonts"
 STR_FONT_SIZE: "Grandària de la lletra (UI)"
 STR_LINE_SPACING: "Interlineat del lector"