feat: register help button in the header

[brian-smith-tcril]

Summary

Companion PR to openedx/frontend-base#251. Registers a help button in the header for instructor-dashboard via the new helpButtonSlotOperation helper from frontend-base.

The URL comes from getAppConfig(instructorDashboardAppId).SUPPORT_URL at render time. In production it's populated via the backend's frontend site config API:

• MFE_CONFIG_OVERRIDES.instructor-dashboard.SUPPORT_URL if set, else
• the help-tokens legacy fallback (resolved per-app on the backend), else
• commonAppConfig.SUPPORT_URL as a final fallback.

If none are set, the button doesn't render — matching the existing learner-dashboard behavior.

This PR depends on openedx/frontend-base#251 — do not merge before that.

Test plan

• nvm use && npm run test — full suite green
• In template-site dev, set commonAppConfig.SUPPORT_URL and verify the Help link appears in the header on instructor-dashboard routes
• Set a per-app override for instructor-dashboard and verify it takes precedence over commonAppConfig
• Verify the link disappears when both are empty (and help-tokens fallback isn't configured in your local backend)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.96%. Comparing base (8532409) to head (eb768b6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/slots.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
+ Coverage   89.89%   89.96%   +0.06%     
==========================================
  Files         139      139              
  Lines        2277     2291      +14     
  Branches      459      495      +36     
==========================================
+ Hits         2047     2061      +14     
+ Misses        224      222       -2     
- Partials        6        8       +2     

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[arbrandes]

👍🏼

[diana-villalvazo-wgu]

Should we ignore slots.tsx on codecov? 🤔