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/js/modes/sstv.js

@@ -1428,9 +1428,13 @@ const SSTV = (function() {
      * Destroy — close SSE stream and clear ISS tracking/countdown timers for clean mode switching.
      */
     function destroy() {
-        if (eventSource) {
-            eventSource.close();
-            eventSource = null;
+        if (isRunning) {
+            stop().catch(() => {});
+        } else {
+            if (eventSource) {
+                eventSource.close();
+                eventSource = null;
+            }
         }
         stopIssTracking();
         stopCountdown();