feat: add experimental Linktree import

[jojo58fr]

Description

Adds an experimental Linktree import flow in Settings, gated behind VITE_ENABLE_IMPORT=true.

This PR:

• adds a new Linktree import action in Settings with explicit experimental messaging
• imports Linktree title, description, profile picture, main links, and supported social accounts into a new Bento
• keeps imported social accounts in the profile without auto-adding them as Bento social blocks
• fixes the builder import flow so imported avatar/bio update immediately after import
• preserves more profile fields when importing Bento JSON
• documents the new VITE_ENABLE_IMPORT environment variable
• fixes existing lint/type-check issues encountered while validating the change

Related Issue

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📚 Documentation update
• 🎨 Style/UI improvement
• ♻️ Code refactoring (no functional changes)
• ⚡ Performance improvement
• 🔧 Configuration/build changes

Checklist

• My code follows the project's code style
• I have run npm run lint and fixed any issues
• I have run npm run format to format my code
• I have run npm run type-check with no errors
• I have tested my changes locally with npm run dev
• I have added/updated documentation if needed
• My changes generate no new warnings

Screenshots (if applicable)

Additional Notes

• The feature is disabled by default and only appears when VITE_ENABLE_IMPORT=true.
• Linktree import relies on parsing public page data and is intentionally labeled experimental because some content may still require manual cleanup after import.