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/memory_test.go

@@ -141,7 +141,6 @@ func TestInMemoryStore_SaveSettings(t *testing.T) {
 	store := NewInMemoryStore()
 
 	settings := &Settings{
-		APIEndpoint:             "http://example.com",
 		ImageEditSelector:       ".image-edit",
 		ImageGenerationSelector: ".image-gen",
 		TextGenerationSelector:  ".text-gen",
@@ -161,7 +160,6 @@ func TestInMemoryStore_GetSettings(t *testing.T) {
 
 	// Set some settings
 	settings = &Settings{
-		APIEndpoint:             "http://example.com",
 		ImageEditSelector:       ".image-edit",
 		ImageGenerationSelector: ".image-gen",
 		TextGenerationSelector:  ".text-gen",
@@ -172,5 +170,5 @@ func TestInMemoryStore_GetSettings(t *testing.T) {
 	// Get the settings
 	settings, err = store.GetSettings()
 	require.NoError(t, err)
-	assert.Equal(t, "http://example.com", settings.APIEndpoint)
+	assert.Equal(t, ".image-edit", settings.ImageEditSelector)
 }