fix(login): clarify that https:// URLs are accepted in server URL field (#171)

https:// was already supported by the code (startsWith('http') check) but
the placeholder text only showed bare host:port examples, giving no hint
that a full URL with protocol is valid.

- Updated serverUrlPlaceholder in login namespace (all 8 locales) to show
  https://music.example.com as the domain example
- Added settings.serverUrlPlaceholder i18n key (all 8 locales) and wired it
  into AddServerForm — previously the placeholder was a hardcoded English string

Closes #171

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/Settings.tsx

@@ -168,7 +168,7 @@ function AddServerForm({ onSave, onCancel }: { onSave: (data: Omit<ServerProfile
       </div>
       <div className="form-group" style={{ marginBottom: '0.75rem' }}>
         <label style={{ fontSize: 13 }}>{t('settings.serverUrl')}</label>
-        <input className="input" type="text" value={form.url} onChange={update('url')} placeholder="192.168.1.100:4533" autoComplete="off" />
+        <input className="input" type="text" value={form.url} onChange={update('url')} placeholder={t('settings.serverUrlPlaceholder')} autoComplete="off" />
       </div>
       <div className="form-row" style={{ marginBottom: '0.75rem' }}>
         <div className="form-group">