docs: Review and conform the Authentication pages, add expired-auth handling (#654)

[developerjamiu]

Phase 2 IA, PR 4 of 7 (tracked in #671): a review and conformance pass over the Authentication pages. Most of this section was already reworked by earlier PRs, so this is fixes and consistency rather than net-new content, plus the expired-auth gap. Legacy pages stay at their current location (archiving is deferred to the final PR of this series).

What changed

• The basics: added "Validate the session and handle expiry" (validateAuthentication / initialize, including how transient failures avoid signing offline users out). Corrected two factual errors: session.authenticated is a synchronous getter (not async), and userIdentifier is a String (the user's UuidValue is authUserId).
• Custom providers (OAuth2 utility): fixed a page-breaking unclosed code fence that collapsed a section and a table into a code block; removed a duplicate heading; corrected int authUserId to UuidValue.
• Custom overrides: fixed a $localhost string-interpolation bug and two code samples that would not compile.
• Setup: version pins 3.x.x to 4.0.0-beta.0 (the modular auth packages are on the 4.0-beta line).
• Firebase: completed the FirebaseServiceAccountCredentials constructor reference (it listed three of four).
• Across the section: house-style sweep (em dashes, banned vocab, hedge words, sentence-leading code spans, grammar, numbered-list and code-sample fixes) and section headings normalized to sentence case.

Verification

Checked against the framework source (modules/serverpod_auth and the core auth package): session.authenticated is a synchronous AuthenticationInfo? getter; userIdentifier is a String; AuthUser.id is a UuidValue; the modular auth packages are 4.0.0-beta.0; validateAuthentication and initialize behave as documented; FirebaseServiceAccountCredentials has four constructors. Load-bearing API symbols were spot-checked and exist. Build passes locally.

Notes

• Folds in the expired-auth half of Document client URL configuration per environment and handling expired auth #654; the client-URL half landed in docs: Reshape Endpoints & APIs concepts, archive deprecated streaming #673. Advances the auth docs epic (epic: Improve the auth docs #503).
• Not included, left for a later pass: Microsoft sign-in (Improve the Microsoft sign-in documentation #508, being reworked separately), net-new Troubleshooting and Publishing-to-production sections for the email, anonymous, and passkey providers, and a set of detailed API-symbol checks (the load-bearing ones were verified).
• Legacy auth pages stay at their current URLs; archiving them is deferred to the final PR of this series.

Closes #654

[Zfinix]

LGTM 🚀