[SETTINGS-001] Persist account settings to durable storage

[chev0004]

Outline

Summary

Persists the settings form to a per-user store so account, appearance, notification, and privacy preferences load on open and survive refresh and new sessions.

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Changes & Enhancements

• Type of Change:
  • Feature
  • Bugfix
  • Refactor
  • Chore
• Changes:
  • (database):
    • Add a user_settings table holding theme, application language, time format, and notification preferences, with migration 0007.
  • (settings):
    • Load email, privacy, theme, language, time format, and notification preferences for the signed-in user.
    • Save every field through a new POST /api/settings endpoint scoped to the current user, with invalid email blocked by inline validation.
    • Show a save error state when persistence fails.

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Page / App Changes

Affected Next.js Routes (Local):

• /[lang]/settings
• /api/settings

Before (Screenshot)

After (Screenshot)

Account: email loaded from the persisted account record.

Appearance: theme, application language, and time format loaded from durable storage.

Privacy: profile visibility toggles loaded from the user's profile.

Notifications: each notification preference persisted independently.

Save error state shown when persistence fails.

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Component / Storybook Changes

Affected Storybook Components (Local):

Before (Screenshot)

After (Screenshot)

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Not Doing

• Saved theme and application language are stored as preferences only; applying them to the live UI appearance and route locale is left for a follow-up.

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Other (Remarks)

Adds database migration 0007; run bun run db:migrate after merge.

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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.