fix: allow non-Latin characters in display names

[culfin]

Problem

Registering with a display name such as Jürgen Müller fails. The form
shows "Erlaubt sind a-z, 0-9, - . _" and the request never succeeds.

Two checks reject it, both relying on \w, which means [A-Za-z0-9_] in
JavaScript as well as in Go:

ui/src/pages/Users/Register/components/SignUpForm/index.tsx:60	const nameRegex = /^[\w.-\s]{2,30}$/
pkg/checker/username.go:25	usernameReg = `^[\w.\- ]{2,30}$`

This affects every language except English — German umlauts, accented
letters in French, Spanish or Portuguese, Cyrillic, Greek. Answer ships
translations for over 40 languages, but a user whose name contains a
letter outside ASCII cannot register under it.

Why the check exists

The second check is not really about the display name. MakeUsername
derives the username from it, and the username appears in a profile URL,
so restricting it makes sense. The restriction is just applied one step
too early — to the display name rather than to the derived username.

Approach

Answer already solves exactly this problem elsewhere: question URLs are
transliterated in pkg/htmltext via unidecode.Unidecode, and
MakeUsername already handles Chinese through pinyin. This change
carries that idea through.

1. Transliterate the display name before deriving the username, using
   github.com/mozillazg/go-unidecode — already a direct dependency.
2. Drop what transliteration may leave behind: Ольга becomes Ol'ga,
   and an apostrophe is not a valid username character.
3. Widen the front-end check to Unicode letters and digits (\p{L},
   \p{N}).

The display name keeps its original spelling. Only the derived username
is transliterated:

Jürgen Müller  ->  jurgen-muller
José García    ->  jose-garcia
Ольга          ->  olga
Ελένη          ->  elene
Straße         ->  strasse
Kerstin Harms  ->  kerstin-harms   (unchanged)

Chinese input is unaffected — pinyin still runs first, and its output
is already ASCII.

Notes

• No new dependency.
• gofmt clean.
• Found while migrating a 26-year-old German optometry forum with 5,479
  member accounts to Answer; roughly one in twenty names carries a
  character outside ASCII.