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/system.css

@@ -17,7 +17,7 @@
     text-transform: uppercase;
     letter-spacing: 0.12em;
     color: var(--accent-cyan, #00d4ff);
-    border-bottom: 1px solid rgba(0, 212, 255, 0.2);
+    border-bottom: 1px solid rgba(var(--accent-cyan-rgb), 0.2);
     padding-bottom: 6px;
     margin-top: 8px;
 }
@@ -221,7 +221,7 @@
     fill: none;
     stroke: var(--accent-cyan, #00d4ff);
     stroke-width: 1.5;
-    filter: drop-shadow(0 0 2px rgba(0, 212, 255, 0.4));
+    filter: drop-shadow(0 0 2px rgba(var(--accent-cyan-rgb), 0.4));
 }
 
 .sys-sparkline-area {