Add RAW boot splash & home background support, expose settings, and draw gear icon in launcher UI

[Inter-Raptor]

Motivation

• Add support for fullscreen RAW images for boot splash and home background configurable from settings.json to improve branding and UX at startup and on the home screen.
• Expose new settings so users can configure splash duration and image paths persistently.
• Replace the textual settings footer button with a compact drawn gear icon to avoid relying on SD-provided assets.

Description

• Added new fields to SystemSettings: boot_splash_ms, boot_splash_raw, and home_bg_raw, and wired them into settings_manager.h and settings_manager.cpp with defaults, JSON serialization, deserialization, clamping, and debug prints.
• Implemented resolveSdPath() helper and updated launcher_ui.cpp to attempt to draw home_bg_raw in drawHomeScreen() using displayDrawRAW(), logging when the file is absent or invalid.
• Show the configured boot splash during launcherInit() by loading boot_splash_raw, delaying for boot_splash_ms when successful, and logging fallback behavior.
• Replaced the footer parameters button text with drawSettingsGearButton() which draws a 16x16 gear bitmap in code.
• Updated README.md with usage notes for the new settings.json keys and details about home screen behavior and game ordering.

Testing

• Performed an automated project build/compile (Arduino toolchain) which completed successfully.
• Exercised settings load/save via automated parsing during build checks which reported the new fields without JSON errors.

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Codex Task