Fix --binary installer failure when no GitHub release exists

[Copilot]

install.ps1 --binary and install.sh --binary blindly hit releases/latest/download/ and abort with a misleading "check your internet connection" error when no release has been published.

Changes

• API pre-check: Query api.github.com/repos/.../releases/latest before attempting download to distinguish "no release exists" from "network error" from "asset missing from release"
• Fallback to source build: On any binary download failure, prompt the user to build from source instead of hard-aborting
• Specific error messages: Three distinct failure paths get distinct messages (no releases, wrong assets, download failure)
• Deduplicated fallback logic: Extracted Fallback-ToSourceBuild (ps1) / fallback_to_source_build (sh) helpers

Before

[error] Failed to download binary from https://github.com/.../pea-windows-x86_64.exe
[error] Check your internet connection or try the source install (without --binary).
# exits 1

After

[error] No releases found for HKTITAN/PeaToPea.
[error] The project has not published a release yet.
[warn]  Pre-built binary is not available.
  Would you like to build from source instead? [y/N]:

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