[QA-001] Add loading, empty, and error states

[chev0004]

Outline

Summary

Rounds out data states on real routes: profile, settings, and saved get route-level loading skeletons matching their layouts, the saved page survives a failed load with an actionable retry state, and Storybook gains failed-save stories for the profile and settings forms.

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Changes & Enhancements

• Type of Change:
  • Feature
  • Bugfix
  • Refactor
  • Chore
• Changes:
  • (Routes):
    • Add loading.tsx skeletons for /profile, /settings, and /saved using the existing shimmer idiom; discovery already streams through its Suspense fallback.
    • The saved page catches load failures and renders an error card with a retry action instead of crashing the route.
  • (Storybook):
    • Add SaveError stories with play assertions for ProfilePage and SettingsPage covering the failed-save banner and preserved dirty state.
  • (i18n):
    • Add saved-page error copy to en and ja.

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Page / App Changes

Affected Next.js Routes (Local):

• /[lang]/profile, /[lang]/settings, /[lang]/saved (loading skeletons)
• /[lang]/saved (load error state with retry)

Before (Screenshot)

After (Screenshot)

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Component / Storybook Changes

Affected Storybook Components (Local):

• Features/Profile/ProfilePage > Save Error
• Features/Settings/SettingsPage > Save Error

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.