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/nl.ts

@@ -312,7 +312,7 @@ export const nlTranslation = {
     serverName: 'Servernaam (optioneel)',
     serverNamePlaceholder: 'Mijn Navidrome',
     serverUrl: 'Server-URL',
-    serverUrlPlaceholder: '192.168.1.100:4533 of music.voorbeeld.nl',
+    serverUrlPlaceholder: '192.168.1.100:4533 of https://music.voorbeeld.nl',
     username: 'Gebruikersnaam',
     usernamePlaceholder: 'admin',
     password: 'Wachtwoord',
@@ -413,6 +413,7 @@ export const nlTranslation = {
     servers: 'Servers',
     serverName: 'Servernaam',
     serverUrl: 'Server-URL',
+    serverUrlPlaceholder: '192.168.1.100:4533 of https://music.voorbeeld.nl',
     serverUsername: 'Gebruikersnaam',
     serverPassword: 'Wachtwoord',
     addServer: 'Server toevoegen',