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/en.ts

@@ -314,7 +314,7 @@ export const enTranslation = {
     serverName: 'Server Name (optional)',
     serverNamePlaceholder: 'My Navidrome',
     serverUrl: 'Server URL',
-    serverUrlPlaceholder: '192.168.1.100:4533 or music.example.com',
+    serverUrlPlaceholder: '192.168.1.100:4533 or https://music.example.com',
     username: 'Username',
     usernamePlaceholder: 'admin',
     password: 'Password',
@@ -416,6 +416,7 @@ export const enTranslation = {
     servers: 'Servers',
     serverName: 'Server Name',
     serverUrl: 'Server URL',
+    serverUrlPlaceholder: '192.168.1.100:4533 or https://music.example.com',
     serverUsername: 'Username',
     serverPassword: 'Password',
     addServer: 'Add Server',