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/fr.ts

@@ -313,7 +313,7 @@ export const frTranslation = {
     serverName: 'Nom du serveur (facultatif)',
     serverNamePlaceholder: 'Mon Navidrome',
     serverUrl: 'URL du serveur',
-    serverUrlPlaceholder: '192.168.1.100:4533 ou music.exemple.com',
+    serverUrlPlaceholder: '192.168.1.100:4533 ou https://music.exemple.com',
     username: 'Nom d\'utilisateur',
     usernamePlaceholder: 'admin',
     password: 'Mot de passe',
@@ -414,6 +414,7 @@ export const frTranslation = {
     servers: 'Serveurs',
     serverName: 'Nom du serveur',
     serverUrl: 'URL du serveur',
+    serverUrlPlaceholder: '192.168.1.100:4533 ou https://music.exemple.com',
     serverUsername: 'Nom d\'utilisateur',
     serverPassword: 'Mot de passe',
     addServer: 'Ajouter un serveur',