Add analytics event tracking for CTAs

[rogerSuperBuilderAlpha]

Path: components/NeedsWorkBanner.tsx, app/hackathons/
Size: M

Problem / Context: "Sign in to X" buttons don't track whether users click them. No metrics on feature adoption.

Proposed scope:

• Create lib/analytics.ts with trackEvent(eventName, properties)
• Track: "sign_in_cta_click", "feature_banner_view", "issue_claimed"
• Use Firebase Analytics underneath
• Document event taxonomy

Acceptance criteria:

• Analytics events tracked for ≥5 CTAs
• Events visible in Firebase Analytics dashboard
• No PII in event properties

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This issue was filed as part of the Sports Hack 2026 contributor audit. Grab it if it looks interesting — merged PRs move you up the ranking for one of the 80 confirmed seats. Before starting, drop a comment so we don't double-assign.