fix(ui): show correct endpoint URL in intro screen for custom Anthropic endpoints (#735)

Previously, the startup intro screen always displayed
'https://api.anthropic.com' as the endpoint for Anthropic provider,
even when a custom endpoint was configured via ANTHROPIC_BASE_URL.

This fix reads ANTHROPIC_BASE_URL from environment and displays the
actual configured endpoint, providing accurate information to users
about where their API requests will be sent (proxy gateways, staging,
custom Anthropic-compatible APIs).

Also adds isLocal detection for local endpoints to show appropriate
visual indicator in the startup banner.

Co-authored-by: Ali Alakbarli <ali.alakbarli@users.noreply.github.com>

src/components/StartupScreen.ts

@@ -153,7 +153,9 @@ function detectProvider(): { name: string; model: string; baseUrl: string; isLoc
   const settings = getSettings_DEPRECATED() || {}
   const modelSetting = settings.model || process.env.ANTHROPIC_MODEL || process.env.CLAUDE_MODEL || 'claude-sonnet-4-6'
   const resolvedModel = parseUserSpecifiedModel(modelSetting)
-  return { name: 'Anthropic', model: resolvedModel, baseUrl: 'https://api.anthropic.com', isLocal: false }
+  const baseUrl = process.env.ANTHROPIC_BASE_URL ?? 'https://api.anthropic.com'
+  const isLocal = isLocalProviderUrl(baseUrl)
+  return { name: 'Anthropic', model: resolvedModel, baseUrl, isLocal }
 }
 
 // ─── Box drawing ──────────────────────────────────────────────────────────────