feat: allow username/email login and add PostHog hooks

[gregagi]

Summary

This PR addresses the open WORK ON THIS Clawrn tasks by shipping:

1. Login identifier supports username or email

   • ACCOUNT_LOGIN_METHODS now allows both username and email.
   • Login input copy updated to "Email or username".
   • Removed hardcoded name="username" override on the login field so allauth receives the proper login field.

2. PostHog integration (optional, env-driven)

   • Added POSTHOG_KEY and POSTHOG_HOST settings/env support.
   • Added analytics_settings context processor.
   • Injected PostHog bootstrap/init snippet into both base templates:
     • frontend/templates/base_landing.html
     • frontend/templates/base_app.html
   • Updated .env.example with PostHog vars.

3. Questions link coverage in base templates

   • Added regression test coverage asserting both base templates include the Questions navigation link.

Tests

Ran targeted tests:

ENVIRONMENT=test SECRET_KEY=test SITE_URL=http://testserver DJANGO_READ_DOT_ENV=0 \
  uv run pytest \
  apps/core/tests/test_views.py::TestLoginView \
  apps/core/tests/test_views.py::TestAnalyticsScripts \
  apps/pages/tests.py::PagesMarkdownEndpointsTestCase::test_base_templates_include_questions_link_in_navigation -q

All passed.