Add DeepSeek/xAI/OpenAI-compatible sentiment providers; custom dark dropdown

Providers (admin-switchable, no redeploy):
- DeepSeek and any OpenAI-compatible endpoint (OpenRouter, Together,
  Groq, local Ollama) via a generic Chat Completions adapter + base_url
- xAI Grok with Live Search (search_parameters web+X, citations) —
  grounded tier alongside OpenAI and Gemini
- DeepSeek / generic compatible endpoints are ungrounded (no web
  search); UI shows an amber warning and labels each provider's grounding
- Optional env fallbacks DEEPSEEK_API_KEY / XAI_API_KEY

UI: replace native <select> (unstyleable white popup on Windows) with a
custom dark Dropdown component everywhere — sentiment provider, scanner
filters, market sort, indicators, admin universe, user role.

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

frontend/src/components/admin/UserTable.tsx

@@ -1,6 +1,7 @@
 import { useState } from 'react';
 import { useUsers, useCreateUser, useUpdateAccess, useResetPassword } from '../../hooks/useAdmin';
 import { SkeletonTable } from '../ui/Skeleton';
+import { Dropdown } from '../ui/Dropdown';
 import type { AdminUser } from '../../lib/types';
 
 export function UserTable() {
@@ -53,10 +54,15 @@ export function UserTable() {
         </div>
         <div className="flex flex-col gap-1.5">
           <label className="text-xs text-gray-400">Role</label>
-          <select value={newRole} onChange={(e) => setNewRole(e.target.value)} className="input-glass px-3 py-2 text-sm">
-            <option value="user">User</option>
-            <option value="admin">Admin</option>
-          </select>
+          <Dropdown
+            value={newRole}
+            onChange={setNewRole}
+            className="w-32"
+            options={[
+              { value: 'user', label: 'User' },
+              { value: 'admin', label: 'Admin' },
+            ]}
+          />
         </div>
         <label className="flex items-center gap-2 text-sm text-gray-300 pb-1">
           <input type="checkbox" checked={newAccess} onChange={(e) => setNewAccess(e.target.checked)}