fix: All-time range fills the date pickers with the DB's first record date

Uses first_timestamp from /api/stats (2018-08-01 fallback) as the start
date and today as the end, instead of clearing the pickers.

src/static/dashboard.html

@@ -1278,6 +1278,7 @@
             if (!response.ok) { strip.style.display = 'none'; return; }
             const stats = await response.json();
             const fmtDate = (value) => new Date(value).toLocaleDateString([], { day: 'numeric', month: 'short', year: 'numeric' });
+            state.dbFirstDate = String(stats.first_timestamp).slice(0, 10); // feeds the All-time date range
             $('db-total').textContent = Number(stats.total_measurements).toLocaleString();
             if (stats.rid_measurements != null) {
                 const fmtNum = (value) => Number(value || 0).toLocaleString();
@@ -1320,15 +1321,13 @@
         if ($('history-range').value === 'custom') return;
         state.useDates = false;
         const hours = Number($('history-range').value);
-        if (hours >= 876000) {
-            $('history-start').value = ''; $('history-end').value = '';
-        } else {
         // Mirror the chosen range into the date pickers (local dates)
         const isoLocal = (d) => `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`;
         const now = new Date();
         $('history-end').value = isoLocal(now);
-            $('history-start').value = isoLocal(new Date(now.getTime() - hours * 3600 * 1000));
-        }
+        $('history-start').value = hours >= 876000
+            ? (state.dbFirstDate || '2018-08-01') // All time: DB's first record
+            : isoLocal(new Date(now.getTime() - hours * 3600 * 1000));
         if (state.selectedStation) loadHistory(state.selectedStation);
         else $('history-status').textContent = 'Select a station first — click one on the map or in the list';
     });