feat: Enter to submit, Shift+Enter for newline in comment editor

[AmethystLiang]

Summary

• Add keyboard shortcut: Enter submits the comment, Shift+Enter inserts a new line
• Add hint text below the textarea explaining the keyboard shortcuts
• Fix pre-existing oxlint curly violations in the file

Closes #48

Test plan

• Open "Edit Comment" dialog on a worktree
• Type text and press Enter — should save and close the dialog
• Type text and press Shift+Enter — should insert a new line
• Verify the hint text "Press Enter to save, Shift+Enter for a new line." appears below the textarea

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