feat(opencode): Phase 1 - Backend data structure expansion for OpenCode support

Add OpenCode as the 4th supported application with additive provider management:

- Add OpenCode variant to AppType enum with all related match statements
- Add enabled_opencode field to McpApps and SkillApps structures
- Add opencode field to McpRoot and PromptRoot
- Add database schema migration v3→v4 with enabled_opencode columns
- Add settings.rs support for opencode_config_dir and current_provider_opencode
- Create opencode_config.rs module for config file I/O operations
- Update all services (proxy, mcp, skill, provider, stream_check) for OpenCode
- Add OpenCode support to deeplink provider and MCP parsing
- Update commands/config.rs for OpenCode config status and paths

Key design decisions:
- OpenCode uses additive mode (no is_current needed, no proxy support)
- Config path: ~/.config/opencode/opencode.json
- MCP format: stdio→local, sse/http→remote conversion planned
- Stream check returns error (not yet implemented for OpenCode)

src-tauri/src/services/stream_check.rs

@@ -185,6 +185,14 @@ impl StreamCheckService {
                 )
                 .await
             }
+            AppType::OpenCode => {
+                // OpenCode doesn't support stream check yet
+                return Err(AppError::localized(
+                    "opencode_no_stream_check",
+                    "OpenCode 暂不支持健康检查",
+                    "OpenCode does not support health check yet"
+                ));
+            }
         };
 
         let response_time = start.elapsed().as_millis() as u64;
@@ -477,9 +485,24 @@ impl StreamCheckService {
             }
             AppType::Gemini => Self::extract_env_model(provider, "GEMINI_MODEL")
                 .unwrap_or_else(|| config.gemini_model.clone()),
+            AppType::OpenCode => {
+                // OpenCode uses models map in settings_config
+                // Try to extract first model from the models object
+                Self::extract_opencode_model(provider).unwrap_or_else(|| "gpt-4o".to_string())
+            }
         }
     }
 
+    fn extract_opencode_model(provider: &Provider) -> Option<String> {
+        let models = provider
+            .settings_config
+            .get("models")
+            .and_then(|m| m.as_object())?;
+
+        // Return the first model ID from the models map
+        models.keys().next().map(|s| s.to_string())
+    }
+
     fn extract_env_model(provider: &Provider, key: &str) -> Option<String> {
         provider
             .settings_config