fix(auth): name the state and the address behind a refused sign-in (E9.5 S4)

"Your account is awaiting admin approval." was true and useless. It never said WHICH mailbox the
account was on, and on 2026-09-03 that silence hid a Gmail alias mismatch for a day: a registration
as np.temu@gmail.com met a Google claim of nptemu@gmail.com, and every screen in the funnel agreed
to say nothing about it.

Every refusal password-login can answer with now uses the codebase's structured {code, reason}
detail, built in one helper. The three that arrive AFTER the password is proven carry the address on
the account plus both gate flags; the two that answer before it (invalid, rate_limited) carry
nothing, so a wrong password and an unknown account stay byte-identical. The frontend renders the
first three as a named screen in place of the form — there is nothing to retype once the password is
correct — with an icon, the two-gate checklist, the address in monospace, and a resend CTA where one
helps. Reporting each gate from its own flag is the payoff of S1: approved-but-unverified is a real
state now, and a screen that assumed a sequence would send that user off to wait for an admin who
has already decided.

The Google access-request notice names the address too, and says so when the request was already on
file under ANOTHER spelling of the same mailbox — the honest version of "we didn't open a second
one", stated only when it applies. It moved from the query string to the fragment: the notice now
carries an address, and a query string lands in every reverse-proxy access log. Same reasoning that
put the verify/reset tokens there, and the same owner already clears it. urlencode, not an f-string,
because URLSearchParams reads a bare + in a fragment as a space and would mangle exactly the
+tag addresses this epic exists to protect.

upsert_pending_invite returns the row it settled on beside the is-new flag, since the row's stored
address is what the notice has to name and what mail will go to.

Deviation from the PendingScreen mockup, deliberately: no "Sign out" button and no separate "Not
your address?" link. The mockup drew the panel in the app shell; the accepted decision (a) keeps the
pending user session-less, so there is no session to end, and the address is named directly above a
"Back to sign in" that answers the same question.

UAT on localdev, both languages: all three refusals with the typed spelling deliberately differing
from the stored one (the panel named the stored one), the resend CTA carrying the typed address into
the resend form, and both access-request banners with a +tag address surviving the fragment round
trip. Throwaway accounts and the language switch reverted.

backend/tests/test_auth_session.py

@@ -134,7 +134,7 @@ def test_an_approved_but_unverified_account_still_cannot_sign_in(client, make_us
     r = _login(client, "notyet@example.com", "attacker-chosen")
 
     assert r.status_code == 403
-    assert "verify" in r.json()["detail"].lower()
+    assert r.json()["detail"]["reason"] == "email_unverified"
 
 
 def test_verifying_afterwards_lets_the_original_password_in(client, db, make_user):