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/zh.ts

@@ -312,7 +312,7 @@ export const zhTranslation = {
     serverName: '服务器名称（可选）',
     serverNamePlaceholder: '我的 Navidrome',
     serverUrl: '服务器地址',
-    serverUrlPlaceholder: '192.168.1.100:4533 或 music.example.com',
+    serverUrlPlaceholder: '192.168.1.100:4533 或 https://music.example.com',
     username: '用户名',
     usernamePlaceholder: 'admin',
     password: '密码',
@@ -409,6 +409,7 @@ export const zhTranslation = {
     servers: '服务器',
     serverName: '服务器名称',
     serverUrl: '服务器地址',
+    serverUrlPlaceholder: '192.168.1.100:4533 或 https://music.example.com',
     serverUsername: '用户名',
     serverPassword: '密码',
     addServer: '添加服务器',