feat: add modlist export to Arma 3 Launcher HTML format

- RenderModlistHTML: generates compatible preset HTML from a modlist
- GET /api/modlists/:id/export: returns downloadable .html file
- Export button on ModlistEditor page
- Skips local mods (no workshop ID), includes only Steam workshop mods

frontend/src/api/client.ts

@@ -64,6 +64,7 @@ export const modlistsApi = {
       return r.json() as Promise<Modlist>
     })
   },
+  exportUrl: (id: string) => `${BASE}/modlists/${id}/export`,
   checkMods: (id: string) => request<ModStatus[]>(`/modlists/${id}/check`),
   downloadMissing: (id: string) => request<{ status: string; count?: number }>(`/modlists/${id}/download-missing`, { method: 'POST' }),
   updateAll: (id: string) => request<{ status: string; count?: number }>(`/modlists/${id}/update-all`, { method: 'POST' }),