refactor: drop mode from frontend types; react to settings changes

src/__tests__/opencode_client.spec.ts

@@ -64,7 +64,6 @@ function mockResponse(overrides: {
 }
 
 const sampleRequest: OpenCodeRequest = {
-  mode: 'edit',
   prompt: 'add type hints',
   context: {
     notebookPath: 'foo.ipynb',
@@ -86,7 +85,6 @@ describe('callOpenCodeEdit', () => {
   it('POSTs to /opencode-bridge/edit with JSON body', async () => {
     const respBody = {
       ok: true,
-      mode: 'edit',
       finalSource: 'def foo(x: int) -> int: return x',
       sessionId: 'sid',
       notebookPath: 'foo.ipynb',