feat(sub): auto-detect subscription format by User-Agent (Updated) (#5826)

* feat(settings): add subscription format controls

* feat(sub): auto-detect subscription formats

* fix(xray): validate balancer regexes before save

* Revert "fix(xray): validate balancer regexes before save"

This reverts commit 8a208ce71b.

* doc(endpoints): align indent spaces

* doc(settings): improve error message formatting in validateSubUserAgentRegex

- Use NewErrorf with proper formatting instead of NewError with string concatenation
- Add comment explaining the rationale for returning original pattern value
- This preserves the intentional design where empty input is stored as empty
  in the DB and inherited as the runtime default at read time

---------

Co-authored-by: Tomilla <5007859+Tomilla@users.noreply.github.com>
Co-authored-by: Sanaei <ho3ein.sanaei@gmail.com>

frontend/src/components/form/index.ts

@@ -1,6 +1,7 @@
 export { default as DateTimePicker } from './DateTimePicker';
 export { default as JsonEditor } from './JsonEditor';
 export { default as HeaderMapEditor } from './HeaderMapEditor';
+export { default as GoRegexInput, validateGoRegex } from './GoRegexInput';
 export { default as SelectAllClearButtons } from './SelectAllClearButtons';
 export { default as RemarkTemplateField } from './RemarkTemplateField';
 export { default as RemarkVarPicker } from './RemarkVarPicker';