Draft: Convert Authentication bootstraps to initVueApp

Stacked on !247299 (merged) (the boring initVueApp wave) — retarget to master when it merges.

The wave defers all files touching the [Authentication] CODEOWNERS section (owner @gitlab-org/software-supply-chain-security/authentication/approvers) into this dedicated MR, so that required approval doesn't gate the rest of the mechanical conversion.

Same single mechanical codemod shape as the wave: new Vue({ el, render }) → initVueApp({ el, component, props, events }). No behavior change on Vue 2 (production).

Files converted:

• app/assets/javascripts/access_tokens/index.js
• app/assets/javascripts/authentication/mount_2fa.js
• app/assets/javascripts/authentication/password/index.js
• app/assets/javascripts/authentication/sessions/index.js
• app/assets/javascripts/authentication/sign_in/index.js
• app/assets/javascripts/authentication/two_factor_auth/index.js
• app/assets/javascripts/authentication/webauthn/index.js
• app/assets/javascripts/authentication/webauthn/registration.js
• app/assets/javascripts/oauth_application/index.js
• ee/app/assets/javascripts/ldap/index.js
• ee/app/assets/javascripts/password/password_validator.js

Verification

• No spec files needed changes (none of these bootstraps had dedicated spec coverage in the wave).
• Diff against the target branch is limited to exactly these 11 files.