# Letterbox Fill: 4-Mode Restructure (Solid, Blended, Gradient, None)

## Task
Restructure the letterbox fill modes from 3 (None, Gradient, Solid) to 4 distinct modes with clearer semantics.

## New Modes
1. **Solid** (new) - Picks the dominant (average) shade from the edge and fills the entire letterbox area with that single dithered color.
2. **Blended** (renamed from old "Solid") - Uses per-pixel sampled edge colors with noise dithering, no distance-based interpolation.
3. **Gradient** - Existing gradient behavior (interpolates per-pixel edge color toward a target color).
4. **None** - No fill.

## Changes Made

### `src/CrossPointSettings.h`
- Updated `SLEEP_SCREEN_LETTERBOX_FILL` enum: `LETTERBOX_NONE=0`, `LETTERBOX_SOLID=1`, `LETTERBOX_BLENDED=2`, `LETTERBOX_GRADIENT=3`.
- Note: enum values changed from the old 3-value layout. Existing saved settings may need reconfiguring.

### `src/SettingsList.h`
- Updated "Letterbox Fill" option labels to: "None", "Solid", "Blended", "Gradient".

### `src/activities/boot_sleep/SleepActivity.cpp`
- `drawLetterboxFill()`: Changed signature from `bool solidFill` to `uint8_t fillMode`. Added dominant shade computation for SOLID mode (averages all edge samples into one value per side). BLENDED and SOLID both skip gradient interpolation; SOLID additionally skips per-pixel edge lookups.
- `renderBitmapSleepScreen()`: Removed `solidFill` bool, passes `fillMode` directly to `drawLetterboxFill`. Updated log message to show the mode name.

## Follow-up Items
- None