[ANALYTICS-002] Add per-user activity stats

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Outline

Summary

Adds a per-user activity stats page so any signed-in user can see their own activity, while keeping identity-revealing engagement insights premium.

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Changes & Enhancements

• Type of Change:
  • Feature
  • Bugfix
  • Refactor
  • Chore
• Changes:
  • (db):
    • Add getUserAnalyticsEventCounts, scoped to the acting user's internal id over a recent window.
  • (activity):
    • Add an activity stats page showing the user's own profile views, username copies, saves, and bumps.
    • Replace stats with an enable prompt when product analytics is opted out.
    • Show a locked teaser for premium who-viewed-you and who-copied-you insights.
  • (routing):
    • Add the auth-guarded /[lang]/activity route.
  • (navbar):
    • Add an optional activity link to the user menu, wired across the signed-in routes.
  • (i18n):
    • Add Activity strings and a user menu label in en and ja.

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Page / App Changes

Affected Next.js Routes (Local):

• /[lang]/activity

Before (Screenshot)

After (Screenshot)

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Component / Storybook Changes

Affected Storybook Components (Local):

• Components/UserMenu > Default

Before (Screenshot)

After (Screenshot)

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Not Doing

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Other (Remarks)

Stacked on feat/analytics-001 (ANALYTICS-001, #80), which is not yet merged and carries the analytics events this PR depends on. Base is set to feat/analytics-001; retarget to develop once #80 merges.

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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.