fix: hide+show cycles no longer accumulate prompt nodes in floating panel

Bug: clicking the floating button after the first time caused the panel
content to multiply — every hide+show cycle appended a new prompt while
the previous one stayed in the DOM.

Root cause: the prompt's Lumino parent is null (it's attached to a raw
HTMLDivElement, not a Widget, so neither appendChild nor
Widget.attach(widget, HTMLElement) sets a Lumino parent). The
defensive DOM cleanup in _hidePrompt now removes the prompt's node
explicitly before nulling the reference, so no residual nodes linger
across show+hide cycles. Also use Widget.attach in _showPrompt for the
before-attach / after-attach lifecycle messages the prompt may rely on.

Added a regression test that toggles 5 times and asserts the panel
contains exactly one .opencode-inline-prompt node.

Files:
- src/components/opencode_floating_panel.ts — defensive cleanup + attach
- src/__tests__/opencode_floating_panel.spec.ts — regression test

src/components/opencode_floating_panel.ts

@@ -255,7 +255,14 @@ export class OpenCodeFloatingPanel extends Widget {
         await this._refreshHistory(this._notebookPath);
       }
     });
-    this._panelEl.appendChild(prompt.node);
+    // Use Widget.attach (not raw appendChild) so Lumino sends the
+    // before-attach / after-attach messages — the prompt relies on
+    // these for isAttached/isVisible tracking. The defensive DOM
+    // cleanup in _hidePrompt handles the case where dispose() leaves
+    // a residual node behind (the panel host is an HTMLElement, not a
+    // Widget, so the Lumino parent stays null and dispose() can't
+    // always clean up the DOM on its own).
+    Widget.attach(prompt, this._panelEl);
     this._prompt = prompt;
 
     // Load the current session's static history before streaming starts.
@@ -284,7 +291,20 @@ export class OpenCodeFloatingPanel extends Widget {
   private _hidePrompt(): void {
     this._closeSse();
     if (this._prompt) {
+      const node = this._prompt.node;
       this._prompt.dispose();
+      // Defensive DOM cleanup: the prompt is attached via raw DOM
+      // (appendChild/Widget.attach with an HTMLElement host), so its
+      // Lumino parent is null. In some scenarios — for example when
+      // the dispose path runs synchronously while another show+hide
+      // cycle is already in flight — the dispose() call does not
+      // remove the node from this._panelEl, leaving a residual node
+      // that the next _showPrompt appends to. Without this defensive
+      // removeChild, the user would see the panel content multiply
+      // with every click.
+      if (node.parentNode === this._panelEl) {
+        this._panelEl.removeChild(node);
+      }
       this._prompt = null;
     }
     this._panelEl.hidden = true;