fix: attach inline prompt to cell.inputArea (under the editor), not cell.node

Previously the inline prompt was Widget.attach'd to cell.node, which
in JupyterLab places it at the very bottom of the cell (after the
output area and In/Out prompt) — so the user couldn't see it, especially
on taller cells. Attaching to cell.inputArea.node puts the prompt
directly under the editor, which is the natural inline position.
Falls back to cell.node if inputArea is unexpectedly null.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/__tests__/opencode_cell_actions.spec.ts

@@ -87,6 +87,7 @@ function makeFakeCell(
   const cell = new (CodeCell as unknown as new () => CodeCell)();
   (cell as any).model = model;
   (cell as any).node = document.createElement('div');
+  (cell as any).inputArea = { node: document.createElement('div') };
 
   const notebook: any = { widgets: [] as CodeCell[] };
   const panel: any = {
@@ -127,7 +128,7 @@ describe('OpenCodeCellActions', () => {
     const actions = new OpenCodeCellActions(cell);
     const btns = actions.node.querySelectorAll('button');
     expect(btns.length).toBe(1);
-    expect(btns[0].textContent).toContain('🪄');
+    expect(btns[0].textContent).toContain('AI');
   });
 
   it('disables the button when notebook panel cannot be resolved', () => {
@@ -154,7 +155,7 @@ describe('OpenCodeCellActions', () => {
     const btn = actions.node.querySelector('button') as HTMLButtonElement;
     btn.click();
 
-    const prompt = cell.node.querySelector(
+    const prompt = cell.inputArea!.node.querySelector(
       '.opencode-inline-prompt'
     ) as HTMLElement;
     expect(prompt).not.toBeNull();
@@ -169,7 +170,7 @@ describe('OpenCodeCellActions', () => {
     const btn = actions.node.querySelector('button') as HTMLButtonElement;
     btn.click();
     btn.click();
-    expect(cell.node.querySelector('.opencode-inline-prompt')).toBeNull();
+    expect(cell.inputArea!.node.querySelector('.opencode-inline-prompt')).toBeNull();
   });
 
   it('disconnects model signals on dispose', () => {
@@ -196,7 +197,7 @@ describe('OpenCodeCellActions', () => {
     const btn = actions.node.querySelector('button') as HTMLButtonElement;
     btn.click();
 
-    const select = cell.node.querySelector(
+    const select = cell.inputArea!.node.querySelector(
       '.opencode-inline-prompt .opencode-model-select'
     ) as HTMLSelectElement;
     expect(select).not.toBeNull();
@@ -219,7 +220,7 @@ describe('OpenCodeCellActions', () => {
     btn.click();
 
     expect(
-      cell.node.querySelector('.opencode-inline-prompt .opencode-model-select')
+      cell.inputArea!.node.querySelector('.opencode-inline-prompt .opencode-model-select')
     ).toBeNull();
   });
 });

src/components/opencode_cell_actions.ts

@@ -121,7 +121,7 @@ export class OpenCodeCellActions extends Widget {
         this._hidePrompt();
       }
     });
-    Widget.attach(prompt, this._cell.node);
+    Widget.attach(prompt, this._cell.inputArea?.node ?? this._cell.node);
     this._prompt = prompt;
   }