feat: differentiated panel error messages & auto-hide desktop config (v2.3.5)

- Add Panel.showConfigError() with amber styling and desktop Settings link
- Propagate `skipped` flag from Finnhub and FIRMS API responses
- Show "API key not configured" on Markets/Heatmap/Commodities/FIRMS panels
  when sidecar returns skipped (missing API key)
- ETF, Stablecoin, MacroSignals panels detect upstream API unavailability
  and show retry message instead of generic "Failed to fetch"
- RuntimeConfigPanel auto-hides when all features are configured
- Bump version to 2.3.5

package.json

@@ -1,7 +1,7 @@
 {
   "name": "world-monitor",
   "private": true,
-  "version": "2.3.4",
+  "version": "2.3.5",
   "type": "module",
   "scripts": {
     "lint:md": "markdownlint-cli2 '**/*.md'",