feat: add cover image functionality for articles

- Implemented `resolveApiURL` function to handle various URL formats.
- Updated `ArticleDetail` and `UpdateArticleRequest` interfaces to include `cover_asset_url`.
- Enhanced `ArticleEditorView` to manage cover image uploads, including a new `CoverPickerModal` component.
- Added cover image requirements logic in `cover-requirements.ts` to enforce platform-specific cover image rules.
- Modified backend services to handle cover image uploads and validations, including checks for required cover images for specific platforms.
- Improved error handling for cover image requirements during article publishing.

server/internal/tenant/app/publish_platforms.go

@@ -78,6 +78,31 @@ func extractPlatformsFromJSONPayload(raw []byte) []string {
 	return nil
 }
 
+func resolveArticleCoverAssetURL(wizardStateJSON []byte) *string {
+	value := strings.TrimSpace(extractStringFromJSONPayload(wizardStateJSON, "cover_asset_url"))
+	if value == "" {
+		return nil
+	}
+	return &value
+}
+
+func extractStringFromJSONPayload(raw []byte, key string) string {
+	if len(raw) == 0 {
+		return ""
+	}
+
+	var payload map[string]interface{}
+	if err := json.Unmarshal(raw, &payload); err != nil {
+		return ""
+	}
+
+	value, ok := payload[key].(string)
+	if !ok {
+		return ""
+	}
+	return strings.TrimSpace(value)
+}
+
 func normalizePlatformsFromValue(raw interface{}) []string {
 	switch typed := raw.(type) {
 	case nil:
@@ -101,7 +126,7 @@ func normalizePlatformsFromValue(raw interface{}) []string {
 	}
 }
 
-func mergeArticleWizardState(raw []byte, title string, platforms []string) ([]byte, error) {
+func mergeArticleWizardState(raw []byte, title string, platforms []string, coverAssetURL *string) ([]byte, error) {
 	state := make(map[string]interface{})
 	if len(raw) > 0 {
 		_ = json.Unmarshal(raw, &state)
@@ -123,5 +148,14 @@ func mergeArticleWizardState(raw []byte, title string, platforms []string) ([]by
 		}
 	}
 
+	if coverAssetURL != nil {
+		trimmedCoverURL := strings.TrimSpace(*coverAssetURL)
+		if trimmedCoverURL == "" {
+			delete(state, "cover_asset_url")
+		} else {
+			state["cover_asset_url"] = trimmedCoverURL
+		}
+	}
+
 	return json.Marshal(state)
 }