Load boot and home background assets with fallback names

[Inter-Raptor]

Motivation

• Users reported boot/home RAW images displayed as corrupted and confusion about filenames and placement, so the launcher must be more explicit and tolerant when loading those assets.
• Ensure full-screen RAW files are rendered using the expected 320x240 dimensions to avoid errors caused by mismatched exports.

Description

• Add boot splash attempt at startup in setup() by trying displayDrawRAW("/boot.raw", 0,0,320,240) with a fallback to "/boot.bg.raw" in RaptorLauncher_V0.9.ino.
• Add drawHomeBackground() to launcher_ui.cpp that attempts multiple candidate filenames for the home background (/home_bg.raw, /home.bg.raw, /home_bg.bmp, /home.bg.bmp) and uses displayDrawRAW or displayDrawBMP accordingly.
• Introduce SCREEN_RAW_W and SCREEN_RAW_H constants set to 320x240 and make drawHomeScreen() use drawHomeBackground() with a plain background fallback if none of the files are found.
• Keep existing behavior for icons and titles unchanged while making root-SD paths explicit to reduce naming/path ambiguity for assets.

Testing

• Ran static repository checks with git diff --check which completed successfully.
• Verified repository status with git status --short and the two files were staged and committed without errors.
• No automated build or hardware rendering tests were executed in CI; runtime visual verification on target hardware is still recommended.

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Codex Task