feat(settings): persist the theme through the settings binding

Keeps one writer for saved state: ui.setTheme() writes through
settings.setTheme() and then reloads the palette C cannot see.

docs/lua-api-parity.md

@@ -32,7 +32,7 @@ strength a status screen wants. Its `state` vocabulary is a subset: crosspoint r
 | Refresh | `gui.refresh(mode)`, `REFRESH_FULL/HALF/FAST` | none | An LCD has no waveform modes. |
 | Colour | `COLOR_*` constants, 4 grey levels | `gui.color(r, g, b)` returning RGB565 | 16-bit colour has too many values to enumerate. |
 | Shapes | `drawRoundedRect` + `fillRoundedRect` | one `gui.roundRect(...)` with gradient and border | Fill and border derive from a single distance field, so their edges cannot disagree. |
-| Themes | none | `settings.getTheme/setTheme`, `/lib/theme.lua` | Colour panel. |
+| Themes | none | `settings.getTheme/setTheme`, `ui.setTheme` | Colour panel. |
 | Rotation | `gui.setOrientation("portrait")` | `settings.setRotation(degrees)` persisted, `gui.setRotation(degrees)` for one frame | This device stores rotation in settings and remaps touch to match. |
 | Clock | nothing exposed; UTC offset is a C++ setting | `sys.isClockSynced`, `settings.getTimezone/setTimezone` with POSIX TZ rules | Timezone here is a stored rule, so `os.date()` returns local time with DST handled by libc. |
 | Launching | launcher is C++ | `sys.launch(path)`, home is a Lua app | The launcher is just another app here, named `home`. |