docs: Fix Flutter client imports, client exception example, and the real-time tutorial for 4.0

[FXschwartz]

• Client lives in client.dart (docs: Flutter snippets import client from main.dart, which no longer exports it #794): rc.2 moved client setup out of main.dart into lib/client.dart with an initializeClient() that main() awaits (refactor: Extract client setup into client.dart in template serverpod#5658). Snippets that imported client from main.dart no longer compiled. Fixed in the build-your-first-app pages, working-with-endpoints, the project tree, and the Pixorama tutorial.
• Provider setup follow-up (docs: Flutter snippets import client from main.dart, which no longer exports it #794): the Google, Apple, Facebook, Firebase, and GitHub setup pages told readers to add initialize*SignIn() in main() after client.auth.initialize(); that call is now in initializeClient(). Updated the instructions and snippets, plus the related Google troubleshooting and customizations notes and the legacy-auth migration guide.
• Client exception example (docs: Client exception handling example does not compile in 4.0 #795): ServerpodClientException is sealed with only message, so the statusCode == -1 example did not compile. The page now describes ServerpodClientHttpException (with ServerpodClientUnknownHttpException for unmapped codes such as 413) and ServerpodClientNetworkException, and the example catches those. Also updated the quoted error in the Google troubleshooting page, since toString() prints the runtime type.
• Real-time tutorial (docs: Real-time tutorial uses the removed --mini flag and stale template paths #797): --mini and the Mini guide are gone. The tutorial now creates the project with the database deselected, removes the template's greetings feature instead of a non-existent example.spy.yaml, and uses one running serverpod start instead of two serverpod generate steps. The create reference intro no longer advertises --template mini.

Markdownlint passes on all changed files except for nine pre-existing findings on the Facebook setup page, outside the edited section.

Fixes #794, fixes #795, fixes #797.