feat: Add AI content generation button to new word form

[toyoshi]

Summary

• Added AI content generation button to the new word creation form
• Previously only available for existing words, now works for both new and existing words
• Users can now click the AI button on new word forms to generate content based on the title

Changes

• Updated _form.html.slim to show AI button for both new and existing words
• Added new ai_generate route and controller action for title-based generation
• Implemented handleAiEditNew() JavaScript function to handle new word AI generation
• Modified form logic to use different onclick handlers based on word persistence

Test plan

• Navigate to new word page (/new)
• Verify AI button appears next to "Content" label
• Enter a title in the title field
• Click the AI button and verify content is generated
• Test that existing word edit forms still work correctly

🤖 Generated with Claude Code