feat(acars): add message translator and ADS-B datalink integration

Add ACARS label translation, message classification, and field parsers
so decoded messages show human-readable descriptions instead of raw
label codes (H1, DF, _d, 5Z, etc.). Integrate translated ACARS
messages into the ADS-B aircraft detail panel and add a live message
feed to the standalone ACARS mode.

- New utils/acars_translator.py with ~50 label codes, type classifier,
  and parsers for position reports, engine data, weather, and OOOI
- Enrich messages at ingest in routes/acars.py with translation fields
- Backfill translation in /adsb/aircraft/<icao>/messages endpoint
- ADS-B dashboard: DATALINK MESSAGES section in aircraft detail panel
  with auto-refresh, color-coded type badges, and parsed field display
- Standalone ACARS mode: scrollable live message feed (max 30 cards)
- Fix default N. America ACARS frequencies to 131.550/130.025/129.125
- Unit tests covering all translator functions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

static/css/modes/acars.css

@@ -89,3 +89,22 @@
     0%, 100% { opacity: 1; box-shadow: 0 0 0 0 rgba(74, 158, 255, 0.7); }
     50% { opacity: 0.6; box-shadow: 0 0 6px 3px rgba(74, 158, 255, 0.3); }
 }
+
+/* ACARS Standalone Message Feed */
+.acars-message-feed {
+    scrollbar-width: thin;
+    scrollbar-color: var(--border-color) transparent;
+}
+.acars-message-feed::-webkit-scrollbar {
+    width: 4px;
+}
+.acars-message-feed::-webkit-scrollbar-thumb {
+    background: var(--border-color);
+    border-radius: 2px;
+}
+.acars-feed-card {
+    transition: background 0.15s;
+}
+.acars-feed-card:hover {
+    background: rgba(74, 158, 255, 0.05);
+}