Add RAW boot splash and home background support; settings and UI updates

[Inter-Raptor]

Motivation

• Provide support for fullscreen RAW images for a boot splash and a home-screen background configurable from settings.json to improve launcher branding and UX.
• Persist new settings and expose defaults so image behavior is configurable and robust across boots.
• Improve launcher visuals by drawing a settings gear icon in the footer and adding stricter RAW validation to avoid corrupted draws.

Description

• Added new settings fields to SystemSettings (boot_splash_raw, boot_splash_ms, boot_splash_w, boot_splash_h, home_bg_raw, home_bg_w, home_bg_h) and wired them through settings_manager for load/save/defaults with clamping and debug prints.
• Implemented boot splash and home background rendering in launcher_ui.cpp by loading RAW images via a resolveSdPath() helper and calling displayDrawRAW() during launcherInit() and drawHomeScreen() respectively, and replaced the footer settings label with a code-drawn gear (drawSettingsGearButton).
• Hardened RAW handling in display_manager.cpp by validating the file size against expected (width * height * 2) RGB565 bytes and failing early with logs if mismatched.
• Added CarteSD/README_SD_IMAGES_LAUNCHER.txt and updated project README.md and example CarteSD/settings.json to document the new RAW image options and usage.

Testing

• No automated tests were run for these changes.

---

Codex Task