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.

backend/internal/store/storage.go

@@ -15,7 +15,6 @@ var _ Store = (*FileStore)(nil)
 
 // Settings represents the application settings
 type Settings struct {
-	APIEndpoint             string `json:"api_endpoint,omitempty"`
 	ImageEditSelector       string `json:"image_edit_selector,omitempty"`
 	ImageGenerationSelector string `json:"image_generation_selector,omitempty"`
 	TextGenerationSelector  string `json:"text_generation_selector,omitempty"`