refactor(auth): merge the duplicate login form and localize the auth screens

`home-login-form.tsx` and `login-form.tsx` were two ~240-line near-identical
components. Delete the former and give `LoginForm` a `variant` prop:

- `variant="page"`   sr-only labels plus the register/forgot footer (/login)
- `variant="compact"` visible labels, no footer (homepage sidebar)

Field ids now come from `useId()`, so the two usages can never collide, and the
hardcoded "Show"/"Hide"/"Loading" strings are translated.

Localization of the login and register screens:

- `home-login-form.tsx` was entirely hardcoded English.
- `passwordStrength()` returned hardcoded "Weak"/"Fair"/"Good"/"Strong".
- `register.ts` returned only English strings. It now returns a
  locale-independent `code` next to the message, and the form renders
  `t(code)` with the English string as a fallback.
- Backfilled the new keys across all 25 locales, plus the login/register
  strings that were still English in most of them. `ar`, `fi` and `ja` had
  their entire login/register namespace in English and are now filled in.
  Locale parity stays at 0 missing keys, as `i18n:check` requires.

Copy that did not match the enforced rules: the UI advertised "min 8 chars"
(EN) / "min 6 tekens" (NL) while registration requires 12 characters plus an
uppercase, a lowercase, a digit and a special character. Corrected in every
locale. `password-reset.ts` enforced only 6 characters and is raised to 12 to
match registration.

Accessibility: `login-form.tsx` had no `<label>`, no `id` and no `required` on
any field. All three are now present, and error banners are announced with
`role="alert"`.

Adds `src/i18n/auth-messages.test.ts`, which asserts every `RegisterErrorCode`
resolves to a non-empty message in all 25 locales; verified it fails when a key
is removed. The existing register tests now also assert the error `code`.

src/components/auth/register-form.tsx

@@ -5,7 +5,11 @@ import Script from "next/script";
 import { signIn } from "next-auth/react";
 import { useTranslations } from "next-intl";
 import { useActionState, useEffect, useRef, useState } from "react";
-import { type RegisterState, register } from "@/actions/register";
+import {
+	type RegisterErrorCode,
+	type RegisterState,
+	register,
+} from "@/actions/register";
 import Link from "@/components/link";
 import { signInWithTransientRetry } from "@/lib/auth/sign-in-retry";
 
@@ -16,7 +20,10 @@ interface RegisterFormProps {
 	nonce?: string;
 }
 
-function passwordStrength(pw: string): {
+function passwordStrength(
+	pw: string,
+	t: (key: string) => string,
+): {
 	score: number;
 	label: string;
 	color: string;
@@ -28,10 +35,10 @@ function passwordStrength(pw: string): {
 	if (/\d/.test(pw)) score += 1;
 	if (/[^a-zA-Z0-9]/.test(pw)) score += 1;
 
-	if (score <= 1) return { score, label: "Weak", color: "#ef4444" };
-	if (score <= 2) return { score, label: "Fair", color: "#f59e0b" };
-	if (score <= 3) return { score, label: "Good", color: "#22c55e" };
-	return { score: 5, label: "Strong", color: "#16a34a" };
+	if (score <= 1) return { score, label: t("strengthWeak"), color: "#ef4444" };
+	if (score <= 2) return { score, label: t("strengthFair"), color: "#f59e0b" };
+	if (score <= 3) return { score, label: t("strengthGood"), color: "#22c55e" };
+	return { score: 5, label: t("strengthStrong"), color: "#16a34a" };
 }
 
 export function RegisterForm({
@@ -46,6 +53,21 @@ export function RegisterForm({
 		RegisterState,
 		FormData
 	>(register, { error: null, ok: false });
+
+	/**
+	 * Prefer the locale-independent `code` so the message follows the visitor's
+	 * language, falling back to the server's English string when a code has no
+	 * translation yet.
+	 */
+	const translateErrorCode = (code: RegisterErrorCode | undefined) => {
+		if (!code) return null;
+		try {
+			return t(code);
+		} catch {
+			return null;
+		}
+	};
+	const errorMessage = error ?? translateErrorCode(state.code) ?? state.error;
 	const [termsAccepted, setTermsAccepted] = useState(false);
 	const [termsError, setTermsError] = useState(false);
 	const termsRef = useRef<HTMLDivElement>(null);
@@ -58,7 +80,7 @@ export function RegisterForm({
 	);
 	const autoLoginStartedRef = useRef(false);
 	const [referralCode, setReferralCode] = useState("");
-	const strength = passwordStrength(password);
+	const strength = passwordStrength(password, t);
 
 	// The invite link is `/register?ref=<username>`; read it client-side so the
 	// attribution travels with the sign-up form without server state.
@@ -186,12 +208,14 @@ export function RegisterForm({
 					borderRadius: "0 0 12px 12px",
 				}}
 			>
-				{(error || state.error) && (
+				{errorMessage && (
 					<div
+						role="alert"
+						aria-live="polite"
 						className="animate-fade-in-up p-3 rounded-lg text-sm font-semibold text-primary-foreground"
 						style={{ backgroundColor: "var(--color-danger)" }}
 					>
-						{error || state.error}
+						{errorMessage}
 					</div>
 				)}