Add graphical signal meter for APRS decoding

Backend changes (routes/aprs.py):
- Remove -q h flag from direwolf to enable audio level output
- Add parse_audio_level() to extract levels from direwolf output
- Add rate-limiting (max 10 updates/sec, min 2-level change)
- Push meter events to SSE queue as type='meter'

Frontend changes:
- Add signal meter widget to APRS sidebar
- Horizontal bar gauge with gradient (green->cyan->yellow->red)
- Numeric level display (0-100)
- "BURST" indicator for levels >70
- Status text (weak/moderate/strong signal)
- "No RF activity" state after 5 seconds of silence
- CSS styles in static/css/modes/aprs.css

Also added UK region to dropdown (same freq as Europe: 144.800)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/css/modes/aprs.css

@@ -46,3 +46,86 @@
 .aprs-stat-label {
     color: var(--text-muted);
 }
+
+/* Signal Meter Styles */
+.aprs-signal-meter {
+    margin-top: 12px;
+    padding: 10px;
+    background: rgba(0,0,0,0.3);
+    border: 1px solid var(--border-color);
+    border-radius: 4px;
+}
+.aprs-meter-header {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    margin-bottom: 8px;
+}
+.aprs-meter-label {
+    font-size: 10px;
+    font-weight: bold;
+    letter-spacing: 1px;
+    color: var(--text-secondary);
+}
+.aprs-meter-value {
+    font-size: 12px;
+    font-weight: bold;
+    font-family: monospace;
+    color: var(--accent-cyan);
+    min-width: 24px;
+}
+.aprs-meter-burst {
+    font-size: 9px;
+    font-weight: bold;
+    color: var(--accent-yellow);
+    background: rgba(255, 193, 7, 0.2);
+    padding: 2px 6px;
+    border-radius: 3px;
+    animation: burst-flash 0.3s ease-out;
+}
+@keyframes burst-flash {
+    0% { opacity: 1; transform: scale(1.1); }
+    100% { opacity: 1; transform: scale(1); }
+}
+.aprs-meter-bar-container {
+    position: relative;
+    height: 16px;
+    background: rgba(0,0,0,0.4);
+    border-radius: 3px;
+    overflow: hidden;
+    margin-bottom: 4px;
+}
+.aprs-meter-bar {
+    height: 100%;
+    width: 0%;
+    background: linear-gradient(90deg,
+        var(--accent-green) 0%,
+        var(--accent-cyan) 50%,
+        var(--accent-yellow) 75%,
+        var(--accent-red) 100%
+    );
+    border-radius: 3px;
+    transition: width 0.1s ease-out;
+}
+.aprs-meter-bar.no-signal {
+    opacity: 0.3;
+}
+.aprs-meter-ticks {
+    display: flex;
+    justify-content: space-between;
+    font-size: 8px;
+    color: var(--text-muted);
+    padding: 0 2px;
+}
+.aprs-meter-status {
+    font-size: 9px;
+    color: var(--text-muted);
+    text-align: center;
+    margin-top: 6px;
+}
+.aprs-meter-status.active {
+    color: var(--accent-green);
+}
+.aprs-meter-status.no-signal {
+    color: var(--accent-yellow);
+}