refactor!: move LLM configuration from in-app settings to CLI/env vars

- Remove `api_endpoint` from Settings model and settings UI
- Add `--llm-endpoint` / `AETHERA_LLM_ENDPOINT` and `--llm-key` /
  `AETHERA_LLM_KEY` CLI flags (endpoint is required)
- Update client constructor to accept API key parameter
- Update tests and documentation to reflect new configuration approach

BREAKING CHANGE: LLM endpoint and key must now be provided via
`AETHERA_LLM_ENDPOINT` and `AETHERA_LLM_KEY` environment variables or
CLI flags instead of the Settings page.

frontend/src/types/index.ts

@@ -38,7 +38,6 @@ export interface Model {
 }
 
 export interface Settings {
-  api_endpoint?: string;
   image_edit_selector?: string;
   image_generation_selector?: string;
   text_generation_selector?: string;