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/pages/MarketPage.tsx

@@ -8,7 +8,7 @@ import { RankingsTable } from '../components/rankings/RankingsTable';
 import { WeightsForm } from '../components/rankings/WeightsForm';
 import { Callout } from '../components/ui/Callout';
 import { Disclosure } from '../components/ui/Disclosure';
-import { Select } from '../components/ui/Field';
+import { Dropdown } from '../components/ui/Dropdown';
 import { PageHeader } from '../components/ui/PageHeader';
 import { SkeletonTable } from '../components/ui/Skeleton';
 import { Tabs } from '../components/ui/Tabs';
@@ -72,16 +72,17 @@ function WatchlistPanel() {
             <AddTickerForm />
             <label className="flex items-center gap-2 text-xs text-gray-400">
               <span>Sort by</span>
-              <Select
+              <Dropdown
                 value={sortMode}
-                onChange={(event) => setSortMode(event.target.value as SortMode)}
-                className="!py-1 !text-xs"
-              >
-                <option value="score_desc">Score (high → low)</option>
-                <option value="score_asc">Score (low → high)</option>
-                <option value="name_asc">Name (A → Z)</option>
-                <option value="name_desc">Name (Z → A)</option>
-              </Select>
+                onChange={(v) => setSortMode(v as SortMode)}
+                className="w-44"
+                options={[
+                  { value: 'score_desc', label: 'Score (high → low)' },
+                  { value: 'score_asc', label: 'Score (low → high)' },
+                  { value: 'name_asc', label: 'Name (A → Z)' },
+                  { value: 'name_desc', label: 'Name (Z → A)' },
+                ]}
+              />
             </label>
           </div>