feat: add OpenCode CLI path override and settings UI

- Add opencodeBinary field to settings and persistence flow
- Introduce OpenCode CLI settings panel with Browse and Save actions

packages/ui/src/lib/desktop.ts

@@ -37,6 +37,8 @@ export type DesktopSettings = {
   darkThemeId?: string;
   lastDirectory?: string;
   homeDirectory?: string;
+  // Optional absolute path to `opencode` binary.
+  opencodeBinary?: string;
   projects?: ProjectEntry[];
   activeProjectId?: string;
   approvedDirectories?: string[];