Feature: Add English Language Support

[PU-MEriksson]

Problem
The Momentum anti-procrastination app is currently only available in Swedish, limiting its accessibility to international users. To make the app more inclusive and reach a broader audience, we need to add English language support.

Proposed Solution
Implement English language support as an alternative to Swedish, allowing users to experience the app in their preferred language.

Acceptance Criteria

• All user interface text is translated to English
• OpenAI API calls can generate plans in English when English is selected
• Language preference is maintained throughout the user session
• Both Swedish and English versions maintain the same functionality
• Form validation messages appear in the selected language

Implementation Details

Frontend Changes Needed:

• Translate all static text in components (Header.jsx, Form.jsx, etc.)
• Update button labels and form placeholders
• Translate radio group options for plan detail levels
• Update error and success messages

Backend Changes Needed:

• Modify API endpoints (generate-plan.js, edit-plan.js) to accept language parameter
• Update OpenAI prompts to generate responses in the requested language
• Ensure Swedish and English prompts produce equivalent quality results