Migration V28: Add preferred_locale field to users table for dialect preferences.

This migration adds support for user-specific language dialect preferences, allowing authenticated users to choose their preferred variant (e.g., en-GB vs en-US) while URLs continue to show simplified base language codes (/en/ instead of /en-US/).

Changes

• Adds preferred_locale column to phoenix_kit_users table (nullable string, size 10)
• Creates index on preferred_locale for potential locale-based queries
• Supports full dialect codes (en-US, es-MX, zh-Hans-CN, etc.)

Requirements

• PostgreSQL database
• PhoenixKit V27 or higher

Purpose

Enable simplified URL structure with dialect preferences:

• URLs show base codes: /en/, /es/, /fr/
• Users can save preferred dialects: "en-GB", "es-MX", "pt-BR"
• Guest users get default dialect mapping (en → en-US)
• Translation system uses full dialect codes internally

Usage

Once migrated, users can update their locale preference:

PhoenixKit.Users.Auth.update_user_locale(user, "en-GB")

When visiting /en/dashboard, the system will:

1. Detect base code "en" from URL
2. Resolve to user's preferred_locale ("en-GB")
3. Use "en-GB" for translations while URL stays /en/

Validation

• Format validation: matches ~r/^[a-z]{2}(-[A-Z]{2})?$/
• Existence validation: must be in predefined language list
• NULL allowed: defaults to system dialect mapping

Size Rationale

• Size 10 supports extended codes like "zh-Hans-CN" (10 chars)
• Covers all standard language-REGION codes (5 chars: "en-US")
• Allows future BCP 47 extensions if needed

Notes

• Idempotent: Safe to run multiple times
• No default value: NULL indicates "use system default"
• Index improves potential analytics queries
• Backward compatible: existing users have NULL (use defaults)