fix: label permanently-mock dashboard widgets as sample data (#73)

[rafaio1]

Fixes #73.

• Add per-widget 'Sample data' badges to contributor earnings chart, contributor activity feed, sponsor spend chart, sponsor spend-by-repo panel, sponsor activity feed, and maintainer activity feed whenever the page is in live-data mode (or always for the server-rendered maintainer dashboard).
• Prevents authenticated users from seeing fabricated personal-looking charts/activity under a misleading page-level 'Live data' badge when no backend endpoint exists yet for those widgets.
• TypeScript compiles cleanly with npx tsc --noEmit.

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