fix: enforce consistent password policy across all auth flows

Replace duplicated inline validation with a shared validatePassword()
utility that checks minimum length (8), rejects repetitive and common
passwords, and requires uppercase, lowercase, a digit, and a special
character.

- Add server/src/services/passwordPolicy.ts as single source of truth
- Apply to registration, password change, and admin create/edit user
  (admin routes previously had zero validation)
- Fix client min-length mismatch (6 vs 8) in RegisterPage and LoginPage
- Add client-side password length guard to AdminPage forms
- Update register.passwordTooShort and settings.passwordWeak i18n keys
  in all 12 locales to reflect the corrected requirements

client/src/pages/LoginPage.tsx

@@ -150,7 +150,7 @@ export default function LoginPage(): React.ReactElement {
       }
       if (mode === 'register') {
         if (!username.trim()) { setError('Username is required'); setIsLoading(false); return }
-        if (password.length < 6) { setError('Password must be at least 6 characters'); setIsLoading(false); return }
+        if (password.length < 8) { setError('Password must be at least 8 characters'); setIsLoading(false); return }
         await register(username, email, password, inviteToken || undefined)
       } else {
         const result = await login(email, password)