Improve Default System Template (#3996)

* Feat [UI/UX] [Constant] [DEFAULT System Template] replace hardcoded

- [+] feat(constant.ts): replace hardcoded OpenAI with dynamic ServiceProvider variable in DEFAULT_SYSTEM_TEMPLATE

* Improve [UI/UX] [Chat] "fillTemplateWith"

- [+] feat(chat.ts): add DEFAULT_MODELS to modelConfig
- [+] fix(chat.ts): replace replaceAll with regex in output string replacement
- [+] refactor(chat.ts): use const instead of let for cutoff variable