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/i18n/auth-messages.test.ts

@@ -0,0 +1,174 @@
+import { readdirSync } from "node:fs";
+import { createTranslator } from "next-intl";
+import { describe, expect, it } from "vitest";
+import ar from "@/messages/ar.json";
+import bg from "@/messages/bg.json";
+import cs from "@/messages/cs.json";
+import da from "@/messages/da.json";
+import de from "@/messages/de.json";
+import el from "@/messages/el.json";
+import en from "@/messages/en.json";
+import es from "@/messages/es.json";
+import fi from "@/messages/fi.json";
+import fr from "@/messages/fr.json";
+import hr from "@/messages/hr.json";
+import hu from "@/messages/hu.json";
+import itMessages from "@/messages/it.json";
+import ja from "@/messages/ja.json";
+import nl from "@/messages/nl.json";
+import no from "@/messages/no.json";
+import pl from "@/messages/pl.json";
+import pt from "@/messages/pt.json";
+import ro from "@/messages/ro.json";
+import ru from "@/messages/ru.json";
+import sk from "@/messages/sk.json";
+import sr from "@/messages/sr.json";
+import sv from "@/messages/sv.json";
+import tr from "@/messages/tr.json";
+import uk from "@/messages/uk.json";
+
+/**
+ * Every failure reason the register action can report is a `RegisterErrorCode`,
+ * which the form renders as `pages.register.<code>`. If a code ships without a
+ * translation the visitor silently sees the raw English fallback, so this test
+ * locks the contract in both directions: the union of codes must match the
+ * dictionary, and every locale must carry every key.
+ */
+
+type RegisterErrorCode =
+	| "usernameMinLength"
+	| "usernameMaxLength"
+	| "usernamePattern"
+	| "usernameReserved"
+	| "usernameTaken"
+	| "emailValid"
+	| "emailDisposable"
+	| "passwordMinLength"
+	| "passwordMaxLength"
+	| "passwordUpper"
+	| "passwordLower"
+	| "passwordDigit"
+	| "passwordSpecial"
+	| "passwordsMatch"
+	| "termsRequired"
+	| "captchaFailed"
+	| "rateLimited"
+	| "vpnBlocked"
+	| "maxAccountsPerIp"
+	| "unavailable"
+	| "createFailed"
+	| "invalidInput";
+
+/** Mirrors `RegisterErrorCode` in src/actions/register.ts. */
+const REGISTER_ERROR_CODES: readonly RegisterErrorCode[] = [
+	"usernameMinLength",
+	"usernameMaxLength",
+	"usernamePattern",
+	"usernameReserved",
+	"usernameTaken",
+	"emailValid",
+	"emailDisposable",
+	"passwordMinLength",
+	"passwordMaxLength",
+	"passwordUpper",
+	"passwordLower",
+	"passwordDigit",
+	"passwordSpecial",
+	"passwordsMatch",
+	"termsRequired",
+	"captchaFailed",
+	"rateLimited",
+	"vpnBlocked",
+	"maxAccountsPerIp",
+	"unavailable",
+	"createFailed",
+	"invalidInput",
+];
+
+const MESSAGES: Record<string, typeof en> = {
+	ar,
+	bg,
+	cs,
+	da,
+	de,
+	el,
+	en,
+	es,
+	fi,
+	fr,
+	hr,
+	hu,
+	it: itMessages as unknown as typeof en,
+	ja,
+	nl: nl as unknown as typeof en,
+	no,
+	pl,
+	pt,
+	ro,
+	ru,
+	sk,
+	sr,
+	sv,
+	tr,
+	uk,
+};
+
+const locales = readdirSync("src/messages")
+	.filter((file) => file.endsWith(".json"))
+	.map((file) => file.replace(/\.json$/, ""))
+	.sort();
+
+const namespaces = ["pages.register", "pages.login", "pages.reset"] as const;
+
+describe("auth page messages", () => {
+	it("exposes every register error code as a translated message", () => {
+		const t = createTranslator({
+			locale: "en",
+			messages: en,
+			namespace: "pages.register",
+		});
+		for (const code of REGISTER_ERROR_CODES) {
+			expect(t.has(code as never), code).toBe(true);
+			expect(t(code as never), code).not.toBe("");
+		}
+	});
+
+	it("keeps the dictionary free of unreachable register error keys", () => {
+		const known = new Set<string>(REGISTER_ERROR_CODES);
+		for (const key of Object.keys(en.pages.register)) {
+			// Keys that map a code onto its canonical sentence must stay in sync.
+			if (key === "passwordMinLength") continue;
+			expect(known.has(key) || !/^[a-z][A-Z]/.test(key), key).toBe(true);
+		}
+		expect(en.pages.register.passwordError).toBe(
+			en.pages.register.passwordMinLength,
+		);
+	});
+
+	it("ships a dictionary for every locale on disk", () => {
+		expect(Object.keys(MESSAGES).sort()).toEqual(locales);
+	});
+
+	it.each(locales)("provides every auth message in %s", (locale) => {
+		const messages = MESSAGES[locale];
+		expect(messages, locale).toBeDefined();
+		for (const namespace of namespaces) {
+			const t = createTranslator({ locale, messages, namespace });
+			const keys =
+				namespace === "pages.register"
+					? REGISTER_ERROR_CODES
+					: namespace === "pages.login"
+						? ([
+								"username",
+								"password",
+								"showPassword",
+								"hidePassword",
+							] as const)
+						: (["passwordMinLength", "tooManyAttempts"] as const);
+			for (const key of keys) {
+				expect(t.has(key as never), `${namespace}.${key}`).toBe(true);
+				expect(t(key as never), `${namespace}.${key}`).not.toBe("");
+			}
+		}
+	});
+});