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/modes/sstv.css

@@ -401,7 +401,7 @@
 .sstv-image-card:hover {
     border-color: var(--accent-cyan);
     transform: translateY(-2px);
-    box-shadow: 0 4px 12px rgba(0, 212, 255, 0.2);
+    box-shadow: 0 4px 12px rgba(var(--accent-cyan-rgb), 0.2);
 }
 
 .sstv-image-card-inner {
@@ -688,7 +688,7 @@
     font-weight: 700;
     color: var(--accent-cyan);
     letter-spacing: 2px;
-    text-shadow: 0 0 20px rgba(0, 212, 255, 0.3);
+    text-shadow: 0 0 20px rgba(var(--accent-cyan-rgb), 0.3);
 }
 
 .sstv-countdown-value.imminent {