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/ticker/IndicatorSelector.tsx

@@ -1,7 +1,7 @@
 import { useState } from 'react';
 import { useQuery } from '@tanstack/react-query';
 import { getIndicator, getEMACross } from '../../api/indicators';
-import { Select } from '../ui/Field';
+import { Dropdown } from '../ui/Dropdown';
 import type { IndicatorResult, EMACrossResult } from '../../lib/types';
 
 const INDICATOR_TYPES = ['ADX', 'EMA', 'RSI', 'ATR', 'volume_profile', 'pivot_points'] as const;
@@ -86,16 +86,12 @@ export function IndicatorSelector({ symbol }: IndicatorSelectorProps) {
       <h3 className="mb-3 text-xs font-medium uppercase tracking-widest text-gray-500">Indicators</h3>
 
       <div className="mb-4">
-        <Select
+        <Dropdown
           value={selectedType}
-          onChange={(e) => setSelectedType(e.target.value)}
-          className="w-full !py-2.5"
-        >
-          <option value="">Select indicator…</option>
-          {INDICATOR_TYPES.map((type) => (
-            <option key={type} value={type}>{type}</option>
-          ))}
-        </Select>
+          onChange={setSelectedType}
+          placeholder="Select indicator…"
+          options={INDICATOR_TYPES.map((type) => ({ value: type, label: type }))}
+        />
       </div>
 
       {selectedType && indicatorQuery.isLoading && (