feat: add sector and product support

[TyceHerrman]

Summary

Add support for selecting SLIDER sectors such as CONUS while preserving the existing full-disk behavior as the default.

Users can now choose a sector with --sector/SATPAPER_SECTOR and optionally override its default product with --product/SATPAPER_PRODUCT. Satellite and sector combinations are validated before downloading imagery.

Changes

• add sector and product configuration through the CLI and environment
• define source-specific tile geometry, zoom levels, products, and composition behavior
• support regional output for GOES CONUS and mesoscale imagery, plus Himawari Japan imagery
• probe for the highest available zoom level before downloading tiles
• reuse the fetched timestamp for tile paths and derive its date locally, avoiding redundant metadata requests
• normalize PNG tile formats through the existing image decoder while preserving alpha-over-black behavior
• document the new options and add SATPAPER_SECTOR to the Docker example
• add coverage for configuration validation, regional geometry, URL construction, zoom fallback, and PNG decoding

Testing

• cargo test --release -- --skip generate_wallpaper — 15 passed
• cargo clippy --all-targets --all-features -- -D warnings
• live --once download for GOES East/CONUS
• live --once download for Himawari/Japan
• invalid satellite/sector and product combinations fail with actionable errors
• git diff --check

Notes

The design was informed by #24 from @karantza.

Closes #23