Use SharathGames1/PollyMC and add dynamic AppImage resolution with robust fallbacks

[FlyingEwok]

Motivation

• Update the installer to use the maintained SharathGames1/PollyMC releases instead of the previous repository and make AppImage acquisition more robust.
• Avoid hardcoded asset names so the installer reliably finds the correct AppImage across release changes and network conditions.

Description

• Replace static PollyMC upstream links in README.md to point at SharathGames1/PollyMC.
• In modules/pollymc_setup.sh add dynamic resolution of the latest AppImage by querying the GitHub Releases API and selecting an AppImage asset with x86_64/amd64 in its name using jq.
• Add fallbacks: pick any AppImage asset if no arch-specific match is found and finally fall back to a legacy static download URL while emitting a warning when API resolution fails.
• Preserve existing behavior on download failure by falling back to PrismLauncher, set USE_POLLYMC appropriately, and make the downloaded AppImage executable.

Testing

• Ran a basic smoke test by invoking the setup_pollymc logic in a network-enabled environment which successfully resolved and downloaded an AppImage and set the USE_POLLYMC flag.
• Verified README link updates render correctly and reference the new SharathGames1/PollyMC repository.
• No additional automated unit tests were added for this change.

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Codex Task