feat: add stream primer help link

[mindrica232-arch]

Closes #353

Summary

• Adds a 'What is a payment stream?' help link to the streams empty state.
• Opens the existing StreamPrimer modal from that link.
• Restores focus to the help link when the primer closes.
• Extends StreamsPageContent tests for the empty-state link and focus-return behavior.

Verification

• npm.cmd test -- app/streams/page.test.tsx: passes, 9 tests.
• npx.cmd eslint app/streams/StreamsPageContent.tsx app/streams/page.test.tsx app/components/EmptyState.tsx app/components/StreamPrimer.tsx: passes.

Notes

• npm.cmd run typecheck is not defined in this repo.
• npx.cmd tsc --noEmit currently fails on existing unrelated repo-wide type errors outside this change.