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>

web/style.css

@@ -199,6 +199,25 @@ body.busy .progress::after {
 .spacer { flex: 1; }
 #count { font-size: 12.5px; font-variant-numeric: tabular-nums; }
 
+/* Export button and menu */
+.export { position: relative; }
+.btn.tool { height: 32px; padding: 0 10px; font-size: 13px; }
+.btn.tool svg { width: 16px; height: 16px; }
+.menu {
+  position: absolute; right: 0; top: calc(100% + 6px); z-index: 40;
+  width: 290px; padding: 6px;
+  background: var(--panel); border: 1px solid var(--border); border-radius: 12px;
+  box-shadow: var(--shadow);
+}
+.menu button {
+  display: flex; flex-direction: column; align-items: flex-start; gap: 1px; width: 100%;
+  padding: 8px 10px; border: 0; border-radius: 8px; background: transparent; text-align: left;
+}
+.menu button:hover, .menu button:focus-visible { background: var(--panel-2); outline: 0; }
+.menu strong { font-size: 13px; font-weight: 600; }
+.menu small { font-size: 12px; color: var(--muted); }
+.menu-note { margin: 4px 0 0; padding: 8px 10px 4px; border-top: 1px solid var(--border); font-size: 11.5px; color: var(--muted); }
+
 /* ---------- Histogram ---------- */
 .histo { padding: 0 20px 6px; }
 .histo .bars {
@@ -441,9 +460,9 @@ select.field:focus { outline: 0; border-color: var(--accent); box-shadow: 0 0 0
   .search { order: 3; flex-basis: 100%; max-width: none; }
   .live .lbl { display: none; }
   .filters { padding: 10px 16px 6px; }
-  .filters select { flex: 1 1 calc(50% - 8px); max-width: none; }
+  .filters select { flex: 1 1 calc(50% - 8px); max-width: none; min-width: 0; }
   .spacer { display: none; }
-  #count { flex-basis: 100%; }
+  #count { flex: 1 1 50%; min-width: 0; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
   .histo { padding: 0 16px 6px; }
   .list, .error-banner { margin-left: 16px; margin-right: 16px; }
   .row {