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>

docker-compose.yml

@@ -17,6 +17,7 @@ services:
       RDNS: ${RDNS:-on}               # replace IP hosts with their DNS name (PTR)
       DNS_SERVER: ${DNS_SERVER:-}     # e.g. 192.168.1.1 to query your LAN DNS; empty = system resolver
       ALLOW_PURGE: ${ALLOW_PURGE:-true}
+      EXPORT_MAX: ${EXPORT_MAX:-100000}
     volumes:
       - logstream-data:/data         # tags.json