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/api/types.ts

@@ -389,6 +389,7 @@ export interface SettingsPayload {
   darkThemeId?: string;
   lastDirectory?: string;
   homeDirectory?: string;
+  opencodeBinary?: string;
   projects?: ProjectEntry[];
   activeProjectId?: string;
   approvedDirectories?: string[];

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[];

packages/ui/src/lib/persistence.ts

@@ -350,6 +350,11 @@ const sanitizeWebSettings = (payload: unknown): DesktopSettings | null => {
     result.homeDirectory = candidate.homeDirectory;
   }
 
+  if (typeof candidate.opencodeBinary === 'string') {
+    const trimmed = candidate.opencodeBinary.trim();
+    result.opencodeBinary = trimmed.length > 0 ? trimmed : undefined;
+  }
+
   const projects = sanitizeProjects(candidate.projects);
   if (projects) {
     result.projects = projects;