fix: seed useUser with auth.currentUser to prevent undefined on initial render (#731)

* fix: seed useUser with auth.currentUser on initial render

Restores synchronous user access that was removed in 4.2.3 (#539).
Without it, child components calling useUser() see status:'loading' /
data:undefined on first render even when the user is already signed in,
because a fresh SuspenseSubject has to wait for the async
onAuthStateChanged callback before emitting.

Only seeds initialData when auth.currentUser is truthy, so the
uninitialized-auth case (currentUser is null before SDK reads from
storage) is not incorrectly treated as signed-out.

Fixes #582

* Update src/auth.tsx

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

* docs: regenerate API reference after useUser line number shift

* docs: regenerate API reference with correct upstream repo source links

* test: add regression test for useUser first-render in useSigninCheck guard

* test: replace guard-pattern test with NEVER-observable regression test

The guard-pattern test passed on main because SuspenseSubject's warmup
subscription made onAuthStateChanged fire before getSnapshot() was called.

The new test installs NEVER as the auth:user observable so it can never emit,
isolating the synchronous-user guarantee to the initialData seeding path only.
Fails on main (status:'loading'), passes with the fix (status:'success').

* fix: return new object from initialData path instead of mutating shared snapshot

useObservable was mutating the object returned by getSnapshot() in-place.
That object is SuspenseSubject._immutableStatus, a single shared reference
across all components using the same observableId. One component's initialData
render would corrupt the snapshot for all other components.

Fixes #736

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Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: tyler-reitz