Adjust sparkline and screenshot timing

Sparklines:
- Poll interval: 2s -> 10s
- History size: 30 -> 180 readings
- Now shows 30 minutes of CPU history

Screenshots:
- Dashboard refresh interval: 15s -> 5s
- Combined with dirty tracking, updates on activity with 5s cap

src/textual_webterm/local_server.py

@@ -788,7 +788,7 @@ class LocalServer:
         function startRefresh() {{
             if (refreshTimer !== null) return;
             refresh();
-            refreshTimer = setInterval(refresh, 15000);
+            refreshTimer = setInterval(refresh, 5000);
         }}
         function stopRefresh() {{
             if (refreshTimer === null) return;