Fix AutoHotkey script handling in setup_nettts_prefix.sh

[h4rm0n1c]

Motivation

• The prefix setup oneliner can fail when flextalk_autoinst.ahk is not present next to the helper script, producing errors like "Script File not found: Z:\home...\flextalk_autoinst.ahk".
• Provide a fallback so the setup helper works when fetched remotely (e.g., via the README oneliner).

Description

• Added DEFAULT_AHK_URL and a --ahk-url option and wired them into the helper selection logic via AHK_URL.
• When flextalk_autoinst.ahk is not found next to the helper, the script now downloads it into $TMPDIR using download_payload and uses the downloaded copy.
• Normalized AHK_SCRIPT_PATH with realpath before converting to a Wine path and invoking AutoHotkey.
• Updated the usage text to document the new --ahk-url override.

Testing

• No automated tests were executed for this change.

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Codex Task