Refactors the Activity page to use Discord's color palette, spacing grid, and component styling, including a more compact header and tab bar, layered cards, and thicker progress bars.
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Refactors the client-side Activity page to use public API endpoints for feed and opportunities, handles authentication errors gracefully, and introduces a new API endpoint `/api/user/set-realm.ts` for updating user realm preferences.
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Update DiscordActivityContext to include an `openExternalLink` function that uses the Discord SDK to open external URLs, replacing `window.open` calls to avoid popup blocking issues within the Discord client.
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