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/commands/config.rs

@@ -51,6 +51,15 @@ pub async fn get_config_status(app: String) -> Result<ConfigStatus, String> {
 
             Ok(ConfigStatus { exists, path })
         }
+        AppType::OpenCode => {
+            let config_path = crate::opencode_config::get_opencode_config_path();
+            let exists = config_path.exists();
+            let path = crate::opencode_config::get_opencode_dir()
+                .to_string_lossy()
+                .to_string();
+
+            Ok(ConfigStatus { exists, path })
+        }
     }
 }
 
@@ -67,6 +76,7 @@ pub async fn get_config_dir(app: String) -> Result<String, String> {
         AppType::Claude => config::get_claude_config_dir(),
         AppType::Codex => codex_config::get_codex_config_dir(),
         AppType::Gemini => crate::gemini_config::get_gemini_dir(),
+        AppType::OpenCode => crate::opencode_config::get_opencode_dir(),
     };
 
     Ok(dir.to_string_lossy().to_string())
@@ -79,6 +89,7 @@ pub async fn open_config_folder(handle: AppHandle, app: String) -> Result<bool,
         AppType::Claude => config::get_claude_config_dir(),
         AppType::Codex => codex_config::get_codex_config_dir(),
         AppType::Gemini => crate::gemini_config::get_gemini_dir(),
+        AppType::OpenCode => crate::opencode_config::get_opencode_dir(),
     };
 
     if !config_dir.exists() {