feat(affiliate): 6 - add referral banner and deeplink handler

[fairlighteth]

Summary

Adds the infrastructure that prepares affiliates for user-facing entry points:

• ReferralNetworkBanner surfaces a Lingui-backed warning while the modal is open but the wallet is disconnected or on an unsupported chain.
• ReferralDeepLinkHandler listens for ?ref=CODE query params, sanitises/normalises them, triggers the referral modal, persists the code, fires analytics, and removes the parameter from the URL.
• referralApi now resolves its base URL via env fallbacks (staging → default) while keeping the timeout configurable.
• Minor shape tweaks to constants/types keep the branch compiling ahead of the UI slices.

Everything stays unmounted for now; integration happens in later PRs.

To Test

1. yarn lint

   • Lint passes; new hooks and components comply with ESLint/TS rules

2. Deep-link smoke test (manual/connect dev tools)

   • Render ReferralProvider + ReferralDeepLinkHandler in Storybook or a sandbox
   • Navigate to /sandbox?ref=MYCODE → Handler sanitises code, opens modal (referral.modalOpen === true), and removes the query param
   • useCowAnalytics().sendEvent receives code_saved with label deeplink

3. Banner behaviour (manual)

   • With the modal open and wallet disconnected, <ReferralNetworkBanner /> renders the warning copy
   • Connecting a supported wallet hides the banner immediately

Background

This slice deliberately ships headless logic so reviewers can validate the deep-link and banner behaviour before the modal UI is mounted in later branches.

[vercel]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/affiliate-ui-6

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