Use a Modal for atl-spam to allow multiline messages

[johnvictorfs]

Motivation

• Slash command text inputs are single-line and previously forced users to type literal \n sequences which were sent raw, so the command needed a native multiline editor for composing spam preview messages.

Description

• Add SpamMessageModal (discord.ui.Modal) with a paragraph TextInput to capture multiline message content natively (bot/cogs/admin.py).
• Rework atl_spam to show the modal first, wait for submission, and then continue the existing preview/confirmation/send flow using the modal's submission interaction.
• Preserve attachment support and confirmation UI while converting literal "\\n" sequences to real line breaks ("\n") for backward compatibility.
• Route user-facing responses (preview, confirmation prompts, progress) through the modal's submitted_interaction to keep interactions consistent.

Testing

• Ran python -m compileall bot/cogs/admin.py which completed successfully.

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Codex Task