Add MarkdownEditor component, asset image upload, and rich description editing

Introduce MarkdownEditor built on @mdxeditor/editor with headings,
lists, links, quotes, image upload with drag-and-drop, and themed CSS
integration. Add asset image upload API (routes, service, storage) and
wire image upload into InlineEditor multiline mode, NewIssueDialog,
NewProjectDialog, GoalDetail, IssueDetail, and ProjectDetail
description fields. Tighten prompt template editor styling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/api/assets.ts

@@ -0,0 +1,14 @@
+import type { AssetImage } from "@paperclip/shared";
+import { api } from "./client";
+
+export const assetsApi = {
+  uploadImage: (companyId: string, file: File, namespace?: string) => {
+    const form = new FormData();
+    form.append("file", file);
+    if (namespace && namespace.trim().length > 0) {
+      form.append("namespace", namespace.trim());
+    }
+    return api.postForm<AssetImage>(`/companies/${companyId}/assets/images`, form);
+  },
+};
+