Add CSV export of the logs matching the current filters

- GET /api/export.csv streams every stored log matching the filters
  (newest first, up to EXPORT_MAX rows, 100000 by default) straight from
  VictoriaLogs, with dates in the time zone chosen in Settings.
- Export button with two variants: CSV (comma, UTF-8) and CSV for Excel
  (semicolon + BOM, formula injection neutralized).
- Store.QueryStream streams query results without buffering them.
- Mobile: filter bar keeps two selects per row, count next to Export.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -13,3 +13,5 @@ RDNS=on
 DNS_SERVER=
 # Allow "Delete all logs" in Settings (true/false)
 ALLOW_PURGE=true
+# Maximum number of rows in a CSV export
+EXPORT_MAX=100000