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.css

@@ -151,3 +151,7 @@ a { color: var(--ui); }
 .nws-dot { display: inline-block; width: 8px; height: 8px; border-radius: 50%; background: #a78bfa; margin-left: 0.35rem; vertical-align: middle; }
 .nws-card .kv b { font-size: 1.05rem; }
 .nws-raw { display: block; margin-top: 0.4rem; font-size: 0.7rem; overflow-wrap: anywhere; color: var(--ink-light); background: var(--paper); border: 1px solid var(--border); border-radius: 8px; padding: 0.35rem 0.55rem; }
+
+/* Performance on phones: blur only the two big fixed bars, lightly */
+.logout, .range-btn, .refresh-btn, .chart-card, .stat, .today-bar { -webkit-backdrop-filter: none !important; backdrop-filter: none !important; }
+.header, .tabbar { -webkit-backdrop-filter: blur(8px); backdrop-filter: blur(8px); }