[SETTINGS-003] Add language display name preference

[chev0004]

Outline

Summary

Adds a settings preference for how language labels render across the app: full localized names like English (the default) or short codes like EN. The choice persists in user settings and applies live to discovery and preview card language pills.

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Changes & Enhancements

• Type of Change:
  • Feature
  • Bugfix
  • Refactor
  • Chore
• Changes:
  • (Database):
    • Add language_display enum (long, short) and column on user_settings, defaulting to long.
  • (Settings):
    • Add a Language Names select to the Appearance section with full-name and short-code options.
    • Persist the preference through /api/settings and load it with the rest of the saved settings.
    • Mirror the saved value to localStorage and dispatch languageDisplayChanged so open cards update immediately.
  • (Discovery):
    • Render ProfileCard language pills as uppercase codes when the preference is short, keeping full localized names otherwise.
  • (i18n):
    • Add setting label, description, and option copy to en and ja.

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Page / App Changes

Affected Next.js Routes (Local):

• /[lang]/settings (appearance section: new Language Names setting)
• /[lang] (discovery card language pills honor the preference)

Before (Screenshot)

After (Screenshot)

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Component / Storybook Changes

Affected Storybook Components (Local):

• Features/Settings/SettingsPage > Default

Before (Screenshot)

After (Screenshot)

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Not Doing

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Other (Remarks)

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Checklists

• Code is well-documented (comments, JSDoc, etc.).
• Existing comments were updated as needed.
• No out-of-scope changes are included.
• Write TODO comments where future work is required.
• Removed unnecessary debug code (e.g., console.log, debugger).
• Self-reviewed and tested locally.