chore(content-server): remove Backbone email-first and index views

[fxa-agent]

Because

• The React app already serves /, /oauth and /authorization. The Backbone index and authorization views are dead code.
• The emailFirstRoutes and simpleRoutes flags are on in stage and prod, so they only keep dead code alive.
• FXA-14379 left some unused code behind.

This pull request

• Changes the (/), authorization(/) and oauth(/) routes in router.js to createReactViewHandler. The (/) route keeps redirect_to. The routes stay because remaining Backbone views navigate to /.
• Deletes views/index.js, views/authorization.js, views/ready.js, the index and ready templates, and the partials brand-messaging, coppa-age-input and third-party-auth.
• Deletes the client helper lib/email-domain-validator.js. The server route validate-email-domain.js stays, because fxa-settings calls it.
• Deletes 11 mixins, the mocha specs of the deleted modules, and the matching lines in test_start.js. The router spec now uses PermissionsView instead of ReadyView for the string-load tests.
• Removes the FXA-14379 leftovers: isPrimaryEmail in verification-reason-mixin.js, PRIMARY_EMAIL_VERIFIED in verification-reasons.ts, and _isVerificationInMobileSafari in app-start.js.
• Sets featureFlagOn: true on emailFirstRoutes and simpleRoutes in react-app/index.js. fullProdRollout: true stays.
• Removes the two config keys from configuration.js, local.json-dist and react-app/types.ts. Removes showReactApp from beta-settings.js and from config.ts in fxa-settings (type and default).
• Removes the emailFirstRoutes clause from cookiesDisabled.spec.ts.
• Removes a stale comment in pocket-migration-mixin.js that names the deleted monitor-client-mixin.

Issue that this pull request solves

Closes: FXA-14378

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.

How to review (Optional)

• Key files/areas to focus on: router.js, react-app/index.js, and the config removals.
• Suggested review order: router and server config first, then the deletions.
• Risky or complex parts: views load by string in router.js. A deleted module that is still in use would show only at run time.

Screenshots (Optional)

No UI change.

Other information (Optional)

Local checks, on the local stack:

• Functional, PASS: tests/signin/signIn.spec.ts ("login as an existing user") and tests/misc/cookiesDisabled.spec.ts.
• Check, PASS: GET /, /oauth and /authorization on port 3030 return 200.
• npx nx build fxa-content-server: PASS.
• npx nx lint fxa-content-server: PASS. This target lints .js files only.
• Type check: PASS for fxa-content-server and fxa-settings.
• Per-file eslint on two .ts files in fxa-content-server: FAIL with a parser error. The package eslint config has no TypeScript parser. The error also occurs on origin/main.

Not run here:

• tests/oauth/*. These need 123done, so CI only.
• The rest of tests/misc, tests/signin and the signup specs.
• The Backbone mocha specs. They do not run in CI.

Follow-ups:

• .circleci/config.yml and the Cloudops config still set REACT_CONVERSION_EMAIL_FIRST_ROUTES and REACT_CONVERSION_SIMPLE_ROUTES. Convict ignores env vars that have no schema entry, so this is safe. Remove them in a follow-up.
• get-index.js still reads emailFirstRoutes.featureFlagOn, which is always true now. I left it as is.

misc-cookiesDisabled-cooki-85178--with-localStorage-disabled-local.webm

misc-cookiesDisabled-cooki-f7f25--with-localStorage-disabled-local.webm

signin-signIn-severity-2-s-e7b7a-n-login-as-an-existing-user-local.webm