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/locales/es.ts

@@ -315,7 +315,7 @@ export const esTranslation = {
     serverName: 'Nombre del Servidor (opcional)',
     serverNamePlaceholder: 'Mi Navidrome',
     serverUrl: 'URL del Servidor',
-    serverUrlPlaceholder: '192.168.1.100:4533 o music.example.com',
+    serverUrlPlaceholder: '192.168.1.100:4533 o https://music.example.com',
     username: 'Usuario',
     usernamePlaceholder: 'admin',
     password: 'Contraseña',
@@ -417,6 +417,7 @@ export const esTranslation = {
     servers: 'Servidores',
     serverName: 'Nombre del Servidor',
     serverUrl: 'URL del Servidor',
+    serverUrlPlaceholder: '192.168.1.100:4533 o https://music.example.com',
     serverUsername: 'Usuario',
     serverPassword: 'Contraseña',
     addServer: 'Agregar Servidor',