Add Playwright E2E coverage for dashboard flows

[saurabhhhcodes]

Summary

• add Playwright configuration and npm scripts for dashboard E2E coverage
• add deterministic mocked E2E tests for the GitHub sign-in entry point, authenticated dashboard widgets, contribution range switching, and goal creation
• add a guarded E2E_TEST_AUTH_BYPASS dashboard path for non-production test runs so CI does not depend on real GitHub OAuth or Supabase state
• wire Playwright into CI and include it in the aggregate CI passed gate

Fixes #236

Testing

• npm run type-check passed
• npm run lint passed with existing warnings in BadgeSection.tsx and CommitTimeChart.tsx
• node /tmp/codex-npm-cli/bin/npm-cli.js exec -- playwright test --list passed and listed 3 tests
• git diff --check passed

Local environment note

• Full npm run test:e2e starts the Next dev server, but this Codex macOS sandbox cannot load Next's native SWC binary because of a local code-signature validation error (ERR_DLOPEN_FAILED). The same repository already shows this SWC issue locally for next build; the new Playwright CI job runs on Ubuntu and installs Chromium explicitly.

GSSoC label request

This PR was opened for the assigned GSSoC issue #236. Please add the scoring labels when reviewing/merging:

• gssoc:approved
• level:advanced
• quality:clean
• type:testing

[vercel]

Someone is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[saurabhhhcodes]

Quick GSSoC label note: if this Playwright E2E coverage PR is accepted, could you please add gssoc:approved, level:advanced, quality:clean, and type:testing?

The only visible remote blocker is Vercel team authorization. Locally, type-check/lint passed, and the test specs compile/list cleanly; the full browser run is blocked in this environment by the native Next SWC code-signing issue noted in the PR.

[Priyanshu-byte-coder]

Closing in favour of #335 — you submitted a newer version of this same E2E test suite. Please make sure #335 incorporates everything from this PR.