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/app/(site)/page.tsx

@@ -6,7 +6,7 @@ import { redirect } from "next/navigation";
 import { getTranslations } from "next-intl/server";
 import type { CSSProperties, ReactNode } from "react";
 import { AnimatedCounter } from "@/components/animated-counter";
-import { HomeLoginForm } from "@/components/auth/home-login-form";
+import { LoginForm } from "@/components/auth/login-form";
 import { Clock } from "@/components/clock";
 import { LanguageSwitcher } from "@/components/language-switcher";
 import Link from "@/components/link";
@@ -870,7 +870,9 @@ export default async function Home() {
 								icon={ICON_NAV_ME}
 								bodyClassName="p-3.5 sm:p-4"
 							>
-								<HomeLoginForm
+								<LoginForm
+									variant="compact"
+									redirectTo="/"
 									captcha={{
 										provider: captcha.provider,
 										siteKey: captcha.siteKey || undefined,