Add ACARS aircraft messaging feature with collapsible sidebar

- Add routes/acars.py with start/stop/stream endpoints for ACARS decoding
- Build acarsdec from source in Dockerfile (not available in Debian slim)
- Add acarsdec installation script to setup.sh for native installs
- Add ACARS to dependency checker in utils/dependencies.py
- Add collapsible ACARS sidebar next to map in aircraft tracking tab
- Add collapsible ACARS panel in ADS-B dashboard with same layout
- Include guidance about needing two SDRs for simultaneous ADS-B + ACARS
- Support regional frequency presets (N.America, Europe, Asia-Pacific)

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

static/css/adsb_dashboard.css

@@ -185,13 +185,138 @@ body {
     position: relative;
     z-index: 10;
     display: grid;
-    grid-template-columns: 1fr 340px;
+    grid-template-columns: 1fr auto 300px;
     grid-template-rows: 1fr auto;
     gap: 0;
     height: calc(100vh - 60px);
     min-height: 500px;
 }
 
+/* ACARS sidebar (between map and main sidebar) - Collapsible */
+.acars-sidebar {
+    background: var(--bg-panel);
+    border-left: 1px solid var(--border-color);
+    display: flex;
+    flex-direction: row;
+    overflow: hidden;
+    transition: width 0.3s ease;
+    width: 280px;
+}
+
+.acars-sidebar.collapsed {
+    width: 32px;
+}
+
+.acars-collapse-btn {
+    width: 32px;
+    min-width: 32px;
+    background: var(--bg-card);
+    border: none;
+    border-right: 1px solid var(--border-color);
+    color: var(--accent-cyan);
+    cursor: pointer;
+    display: flex;
+    flex-direction: column;
+    align-items: center;
+    justify-content: center;
+    gap: 8px;
+    padding: 10px 0;
+    transition: background 0.2s;
+}
+
+.acars-collapse-btn:hover {
+    background: rgba(74, 158, 255, 0.1);
+}
+
+.acars-collapse-label {
+    writing-mode: vertical-rl;
+    text-orientation: mixed;
+    font-size: 10px;
+    font-weight: 600;
+    letter-spacing: 2px;
+    text-transform: uppercase;
+}
+
+.acars-sidebar.collapsed .acars-collapse-label {
+    display: block;
+}
+
+.acars-sidebar:not(.collapsed) .acars-collapse-label {
+    display: none;
+}
+
+#acarsCollapseIcon {
+    font-size: 10px;
+    transition: transform 0.3s;
+}
+
+.acars-sidebar.collapsed #acarsCollapseIcon {
+    transform: rotate(180deg);
+}
+
+.acars-sidebar-content {
+    flex: 1;
+    display: flex;
+    flex-direction: column;
+    overflow: hidden;
+    min-width: 0;
+}
+
+.acars-sidebar.collapsed .acars-sidebar-content {
+    display: none;
+}
+
+.acars-sidebar .panel {
+    flex: 1;
+    display: flex;
+    flex-direction: column;
+    border: none;
+    border-radius: 0;
+}
+
+.acars-sidebar .panel::before {
+    display: none;
+}
+
+.acars-sidebar .acars-messages {
+    flex: 1;
+    overflow-y: auto;
+    min-height: 0;
+}
+
+.acars-sidebar .acars-btn {
+    background: var(--bg-card);
+    border: 1px solid var(--accent-cyan);
+    color: var(--accent-cyan);
+    padding: 6px 10px;
+    font-size: 10px;
+    font-weight: 600;
+    cursor: pointer;
+    transition: all 0.2s;
+    text-transform: uppercase;
+    letter-spacing: 1px;
+}
+
+.acars-sidebar .acars-btn:hover {
+    background: rgba(74, 158, 255, 0.2);
+}
+
+.acars-message-item {
+    padding: 8px 10px;
+    border-bottom: 1px solid var(--border-color);
+    font-size: 10px;
+    animation: fadeIn 0.3s ease;
+}
+
+.acars-message-item:hover {
+    background: rgba(74, 158, 255, 0.05);
+}
+
+@keyframes fadeIn {
+    from { opacity: 0; transform: translateY(-5px); }
+    to { opacity: 1; transform: translateY(0); }
+}
+
 /* Panels */
 .panel {
     background: var(--bg-panel);
@@ -228,8 +353,14 @@ body {
 .panel-indicator {
     width: 6px;
     height: 6px;
-    background: var(--accent-cyan);
+    background: var(--text-dim);
     border-radius: 50%;
+    opacity: 0.5;
+}
+
+.panel-indicator.active {
+    background: var(--accent-green);
+    opacity: 1;
     animation: blink 1s ease-in-out infinite;
 }
 
@@ -656,8 +787,20 @@ body {
     opacity: 0.5;
 }
 
-/* Responsive */
-@media (max-width: 1000px) {
+/* Responsive - medium screens (hide ACARS sidebar, keep main sidebar) */
+@media (max-width: 1200px) {
+    .dashboard {
+        grid-template-columns: 1fr 300px;
+        grid-template-rows: 1fr auto;
+    }
+
+    .acars-sidebar {
+        display: none;
+    }
+}
+
+/* Responsive - small screens (single column) */
+@media (max-width: 900px) {
     .dashboard {
         grid-template-columns: 1fr;
         grid-template-rows: 1fr auto auto;
@@ -667,6 +810,10 @@ body {
         min-height: 400px;
     }
 
+    .acars-sidebar {
+        display: none;
+    }
+
     .sidebar {
         grid-column: 1;
         grid-row: 2;