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/TickerUniverseBootstrap.tsx

@@ -5,6 +5,7 @@ import {
   useUpdateTickerUniverseSetting,
 } from '../../hooks/useAdmin';
 import type { TickerUniverse } from '../../lib/types';
+import { Dropdown } from '../ui/Dropdown';
 
 const UNIVERSE_OPTIONS: Array<{ value: TickerUniverse; label: string }> = [
   { value: 'sp500', label: 'S&P 500' },
@@ -50,18 +51,11 @@ export function TickerUniverseBootstrap() {
       <div className="grid gap-4 md:grid-cols-3">
         <label className="block space-y-1 md:col-span-2">
           <span className="text-xs text-gray-400">Default Universe</span>
-          <select
+          <Dropdown
             value={universe}
-            onChange={(e) => setUniverse(e.target.value as TickerUniverse)}
-            className="w-full input-glass px-3 py-2 text-sm"
-            disabled={isLoading || updateDefault.isPending || bootstrap.isPending}
-          >
-            {UNIVERSE_OPTIONS.map((option) => (
-              <option key={option.value} value={option.value}>
-                {option.label}
-              </option>
-            ))}
-          </select>
+            onChange={(v) => setUniverse(v as TickerUniverse)}
+            options={UNIVERSE_OPTIONS.map((o) => ({ value: o.value, label: o.label }))}
+          />
         </label>
 
         <label className="flex items-end gap-2 pb-2">