chore: improve invitation flow, update project dependencies and documentation for Next.js 16 (#408)

* chore: update project dependencies and documentation for Next.js 16

- Upgraded Next.js from version 15 to 16 across various documentation files and components.
- Updated references to Next.js 16 in AGENTS.md and CLAUDE.md for consistency.
- Incremented application version to 2.21.0 in package.json.
- Refactored identity setup components to improve user experience and added confirmation dialogs for authentication methods.
- Enhanced invitation flow with new logic for handling user redirection and token generation.

* refactor: streamline invitation flow in e2e tests

- Simplified the invitation flow test by using a predefined email instead of generating a random one.
- Removed unnecessary steps such as clearing cookies and reloading the page before user sign-up.
- Enhanced clarity by eliminating commented-out code related to identity verification and user membership checks.

* refactor: improve code readability in IdentitiesPage and UpdatePasswordForm components

- Enhanced formatting of JSX elements in IdentitiesPage and UpdatePasswordForm for better readability.
- Adjusted indentation and line breaks to maintain consistent coding style across components.

* refactor: enhance LinkAccountsList component with user redirection logic

- Updated the LinkAccountsList component to include a redirectToPath option in the useLinkIdentityWithProvider hook for improved user experience.
- Removed redundant user hook declaration to streamline the code structure.

* refactor: update account setup logic in JoinTeamAccountPage

- Introduced a check for email-only authentication support to streamline account setup requirements.
- Adjusted the conditions for determining if a new account should set up additional authentication methods, enhancing user experience for new users.

apps/web/public/locales/en/auth.json

@@ -80,6 +80,8 @@
   "existingAccountHint": "You previously signed in with <method>{{method}}</method>. <signInLink>Already have an account?</signInLink>",
   "linkAccountToSignIn": "Link account to sign in",
   "linkAccountToSignInDescription": "Add one or more sign-in methods to your account",
+  "noIdentityLinkedTitle": "No authentication method added",
+  "noIdentityLinkedDescription": "You haven't added any authentication methods yet. Are you sure you want to continue? You can set up sign-in methods later in your personal account settings.",
   "errors": {
     "Invalid login credentials": "The credentials entered are invalid",
     "User already registered": "This credential is already in use. Please try with another one.",