fix: resolve two-window hang and sweep UI/theming updates

Fix app becoming unresponsive when two browser windows are open: the
root cause was HTTP/1.1 connection pool exhaustion (6-connection limit
per origin). VoiceAlerts was opening 3 SSE streams per window by
default, so two windows produced 8 connections and permanently starved
all regular HTTP requests.

- voice-alerts.js: default all streams to false (opt-in) to stay within
  the browser connection limit; existing user preferences in localStorage
  are preserved
- routes/alerts.py: replace direct AlertManager.stream_events() with
  sse_stream_fanout so both windows receive every alert instead of
  competing for the same queue
- routes/bluetooth_v2.py: same fanout fix via subscribe_fanout_queue,
  preserving named SSE events (device_update, scan_started, etc.)

Also includes accumulated UI/theming changes: accent-cyan CSS variable
sweep across mode CSS/JS files, standalone dashboard pages, template
updates, satellite TLE data refresh, and tile provider default rename.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/css/adsb_history.css

@@ -721,3 +721,28 @@ body {
         grid-template-columns: 1fr;
     }
 }
+
+html[data-ui-tier="enhanced"] {
+    --accent-cyan: #2e7d8a;
+    --accent-cyan-rgb: 46, 125, 138;
+    --border-color: rgba(46, 125, 138, 0.18);
+    --border-glow: rgba(46, 125, 138, 0.20);
+    --grid-line: rgba(46, 125, 138, 0.07);
+    --bg-dark: #000000;
+    --bg-panel: #020404;
+    --bg-card: #020404;
+}
+
+html[data-ui-tier="enhanced"] body {
+    background: #000000;
+}
+
+html[data-ui-tier="enhanced"] .session-strip {
+    background: linear-gradient(120deg, rgba(4, 8, 8, 0.95), rgba(6, 10, 10, 0.95));
+}
+
+html[data-ui-tier="lean"] {
+    --border-color: #2a2a2a;
+    --border-glow: transparent;
+    --grid-line: transparent;
+}