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/lib/types.ts

@@ -165,10 +165,14 @@ export interface ActivationConfig {
 export interface SentimentProviderConfig {
   provider: string;
   model: string;
+  base_url: string;
   api_key_configured: boolean;
   api_key_source: 'database' | 'environment' | 'none';
+  web_search: boolean;
   valid_providers: string[];
   default_models: Record<string, string>;
+  web_search_providers: string[];
+  custom_base_url_providers: string[];
 }
 
 export interface SentimentTestResult {