Performance: stop iPhone freezes (blur over live map, runaway animation, eager charts)

Main app
- No backdrop-filter over the live map, animated sky, or sticky headings (it re-blurs every
  frame; on iPhone that is a classic stutter/lock-up); top/bottom bars keep a light 8px blur
- Sky effect only draws while the hero is on screen and the Now tab is active, ~30fps cap,
  fewer particles on phones; fixed a stale animation-frame id that could stop restarts

Dashboard
- Charts are built only when the Charts tab is opened (was ~11 charts on every page load),
  and the 2-minute refresh no longer rebuilds them in the background
- On phones: max 360 points per chart (was 720), no entry animation, device pixel ratio capped at 2
- Lighter header blur, none on small buttons/cards

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

app/public/dashboard-ui.js

@@ -51,6 +51,7 @@ function setTab(name, updateHash = true) {
 }
 function activeTab() { return document.querySelector('.tab.active')?.dataset.tab || 'now'; }
 function renderTab(name) {
+  if (name === 'charts' && typeof chartsDirty !== 'undefined' && chartsDirty && typeof loadHistory === 'function') loadHistory(currentRange);
   if (name === 'rain' && !rendered.rain) { renderRain(); rendered.rain = true; }
   if (name === 'records' && !rendered.cal) { renderCalendar(); rendered.cal = true; }
   if (name === 'insights' && !rendered.insights) { renderInsights(); rendered.insights = true; }