fix(dashboard): portal NewAgentDialog so it escapes the AgentsView stacking context

On mobile the dialog was rendering with its top hidden behind the in-page
\"Agents\" header. The dialog wasn't actually positioned wrong — its overlay
is `position: fixed; inset: 0`. The problem was that NewAgentDialog
rendered as a child of `.agents-view` (which has `overflow: hidden` and
`position: relative`), and an ancestor stacking context combined with the
relative+overflow parent prevented the fixed overlay from escaping above
the page header in the painter's order.

Render the dialog through `createPortal(..., document.body)` so the
overlay attaches at the document root, escaping every parent stacking
context. This is the standard React modal pattern.

All 9639 dashboard tests still pass — React Testing Library queries
traverse portals transparently.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/NewAgentDialog.tsx

@@ -1,5 +1,6 @@
 import "./NewAgentDialog.css";
 import { useState, useEffect, useCallback } from "react";
+import { createPortal } from "react-dom";
 import type { Agent, AgentCapability, ModelInfo, AgentGenerationSpec, PluginRuntimeInfo } from "../api";
 import { createAgent, fetchAgents, fetchModels, updateGlobalSettings } from "../api";
 import * as apiModule from "../api";
@@ -358,7 +359,13 @@ export function NewAgentDialog({ isOpen, onClose, onCreated, projectId }: NewAge
     </>
   );
 
-  return (
+  // Render through a portal to document.body so the overlay escapes any
+  // ancestor stacking context / `overflow: hidden`. Without this, `position:
+  // fixed` on the overlay was being trapped under .agents-view, so the
+  // dialog rendered with its top hidden behind the in-page Agents header on
+  // mobile (the header isn't taller than the dialog top — it's just stacked
+  // above it because the dialog couldn't escape its container).
+  return createPortal(
     <div className="agent-dialog-overlay" onClick={(e) => { if (e.target === e.currentTarget) handleClose(); }}>
       <div className="agent-dialog" role="dialog" aria-modal="true" aria-label="Create new agent">
         {/* Header */}
@@ -778,6 +785,7 @@ export function NewAgentDialog({ isOpen, onClose, onCreated, projectId }: NewAge
         onGenerated={handleGenerated}
         projectId={projectId}
       />
-    </div>
+    </div>,
+    document.body,
   );
 }