Simplify dashboard auth to GitHub OAuth only

[pzep1]

Summary

• Sign-in and sign-up pages now use GitHub OAuth only; email/password forms and MFA/email verification steps are removed.
• OAuth button block no longer shows a redundant "or" divider after password fields were removed.
• Dashboard shell gets an SVG favicon (/favicon.svg) referenced from index.html.

Test plan

• Run cd dashboard && npm run dev and open /sign-in and /sign-up
• Confirm only GitHub OAuth is offered and redirect after auth still works
• Confirm favicon appears in the browser tab
• Verify Clerk GitHub OAuth is enabled for the dashboard Clerk application

Made with Cursor

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Summary by cubic

Move the dashboard to GitHub OAuth-only authentication. Removes email/password (and MFA/email verification) flows, cleans up the OAuth UI, and adds an SVG favicon.

• Migration
  • Enable GitHub OAuth for the dashboard app in your Clerk project.

^{Written for commit 68d2dbf. Summary will update on new commits. Review in cubic}