Consolidate remaining backend translations

[adi-herwana-nus]

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[Copilot]

Pull request overview

This PR consolidates backend translation files for Chinese (zh), Korean (ko), and English (en) locales by merging scattered YAML files into centralized structures. The changes improve maintainability by organizing translations into fewer, more cohesive files.

Changes:

• Consolidated activerecord error messages and attributes into unified errors.yml and attributes.yml files
• Merged mailer and notifier translations into a central mailers.yml file
• Created csv.yml for CSV export headers and values
• Updated application code references to use the new translation key paths

Reviewed changes

Copilot reviewed 300 out of 343 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
config/locales/{zh,ko,en}/activerecord/errors.yml	Consolidates all activerecord error messages from multiple separate files
config/locales/{zh,ko,en}/activerecord/attributes.yml	Consolidates all activerecord attribute translations from multiple files
config/locales/{zh,ko,en}/mailers.yml	Merges course mailers, instance mailers, and notifiers into one file
config/locales/{zh,ko,en}/csv.yml	Centralizes all CSV export-related translations
config/locales/{zh,ko,en}/errors.yml	Creates unified error messages for authentication, course, and user modules
app/views/**/*.{slim,erb}	Updates translation key references to new consolidated paths
app/controllers/**/*.rb	Updates translation key references in controllers
app/services/**/*.rb	Updates translation key references in services
app/models/**/*.rb	Updates translation key references in models
app/helpers/**/*.rb	Updates translation key references in helpers

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