Build the Analytics module scaffold behind the existing feature flag

[Lakes41]

Difficulty: Advanced
Type: feature

Background
The README already defines a NEXT_PUBLIC_FEATURE_ANALYTICS flag (defaulting to false in every environment, since the module is "not yet built"), and documents the exact process for adding new feature-gated modules via lib/features.ts and <FeatureGate>.

Problem
The Analytics module has a reserved flag and nav slot but no implementation, meaning there's no way to see engagement/membership trends for a community, and no established data-source abstraction for what will eventually need to consume analytics data (from guildpass-core or elsewhere).

Expected outcome
A working Analytics module (behind the existing flag, off by default in prod) renders at least: membership growth over time, role distribution, and gated-resource access attempts (allowed vs. denied), using a pluggable data-source interface so it can start on mock data and later swap to a real analytics backend.

Suggested implementation

• Add the /analytics route wrapped in <FeatureGate enabled={features.analytics} name="Analytics">, following the documented pattern for adding new flagged modules exactly.
• Define an AnalyticsDataSource interface (getMembershipTrend(), getRoleDistribution(), getAccessAttempts()) with a mock implementation seeded from existing mock data patterns in lib/api/mock.ts.
• Build the three chart/summary views using a lightweight charting approach consistent with the project's minimal shadcn-style UI primitives.
• Update .env.example to note the flag's purpose and default, consistent with existing flag documentation style.

Acceptance criteria

• Module is fully gated behind NEXT_PUBLIC_FEATURE_ANALYTICS, hidden from nav and showing "Feature unavailable" when disabled and visited directly
• All three views render correctly against mock data
• Data-source interface is swappable without touching the view components
• Follows the README's documented "Adding a new flag" steps exactly

Likely affected files/directories

• app/analytics/ (new)
• lib/features.ts
• lib/api/mock.ts
• .env.example

[EngrEOOnoja]

I'd like to take this on. I'll add the /analytics route behind the existing feature flag following the documented gating pattern exactly, define a swappable AnalyticsDataSource interface with a mock implementation seeded from existing patterns, and build the three views (membership growth, role distribution, access attempts) so the data source can later swap to a real backend without touching the components.

[grantfox-oss]

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Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #333)
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