Rename the Fly-mode Warnings tab to System Messages, log connection/upload events

Renames DroneEvent/DroneEventSeverity/droneEventLogProvider/
DroneStatusWarningsPanel to SystemMessage/SystemMessageSeverity/
systemMessageLogProvider/DroneStatusMessagesPanel throughout, matching
what the tab now actually shows - general system messages, not just
drone-health warnings.

Adds a new SystemMessageSeverity.info level (blue) for messages that
aren't a warning/error, and three new message sources on top of the
existing battery/failsafe detection:
- "Connected", logged the first time telemetryProvider produces a frame
  after having none - the mirror image of the existing "Connection lost"
  detection (which already fires on the first stream error after having
  had data), so no new transport-specific dependency was needed.
- "Connection type changed to X", from watching connectionSettingsProvider
  (skips the initial load so it doesn't fire on every app start).
- "Mission sent (N waypoints)" / "Mission upload failed: ...", logged
  from FlyScreen's send handler via a new public log() method on the
  notifier, alongside the existing snackbar.

Verified on the Pixel_10a emulator: entering Fly mode logs "Connected"
once the mock telemetry starts, and tapping the send button logs
"Mission sent (3 waypoints)" right after the snackbar.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/test/domain/system_message_test.dart

@@ -0,0 +1,112 @@
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:dmc_app/domain/telemetry/system_message.dart';
+import 'package:dmc_app/transport/flight_controller_link.dart';
+
+TelemetryFrame _frame({int batteryPercent = 80, int navMode = 0}) {
+  return TelemetryFrame(
+    lat: 52.5,
+    lon: 13.4,
+    hasFix: true,
+    numSat: 10,
+    altitudeM: 100,
+    speedMs: 15,
+    headingDeg: 0,
+    armed: true,
+    batteryPercent: batteryPercent,
+    batteryVoltage: 22.0,
+    currentA: 10.0,
+    linkQuality: 90,
+    snrDb: 5,
+    hdop: 1.0,
+    navMode: navMode,
+    flightMode: 'Idle',
+  );
+}
+
+void main() {
+  final now = DateTime(2026, 1, 1, 12, 0, 0);
+
+  group('detectSystemMessages - battery', () {
+    test('kein Ereignis ohne vorherigen Frame', () {
+      final events = detectSystemMessages(
+        previous: null,
+        current: _frame(batteryPercent: 15),
+        now: now,
+      );
+      expect(events, isEmpty);
+    });
+
+    test('kein Ereignis, solange keine Schwelle unterschritten wird', () {
+      final events = detectSystemMessages(
+        previous: _frame(batteryPercent: 80),
+        current: _frame(batteryPercent: 75),
+        now: now,
+      );
+      expect(events, isEmpty);
+    });
+
+    test('"Battery low" beim Unterschreiten von 30%', () {
+      final events = detectSystemMessages(
+        previous: _frame(batteryPercent: 32),
+        current: _frame(batteryPercent: 28),
+        now: now,
+      );
+      expect(events, hasLength(1));
+      expect(events.single.severity, SystemMessageSeverity.warning);
+      expect(events.single.message, contains('Battery low'));
+    });
+
+    test('"Battery critical" beim Unterschreiten von 20%', () {
+      final events = detectSystemMessages(
+        previous: _frame(batteryPercent: 22),
+        current: _frame(batteryPercent: 18),
+        now: now,
+      );
+      expect(events, hasLength(1));
+      expect(events.single.severity, SystemMessageSeverity.critical);
+      expect(events.single.message, contains('Battery critical'));
+    });
+
+    test('nur "Battery critical", wenn beide Schwellen im selben Schritt fallen', () {
+      final events = detectSystemMessages(
+        previous: _frame(batteryPercent: 35),
+        current: _frame(batteryPercent: 15),
+        now: now,
+      );
+      expect(events, hasLength(1));
+      expect(events.single.message, contains('Battery critical'));
+    });
+
+    test('kein erneutes Ereignis, solange der Ladestand unter der Schwelle bleibt', () {
+      final events = detectSystemMessages(
+        previous: _frame(batteryPercent: 25),
+        current: _frame(batteryPercent: 22),
+        now: now,
+      );
+      expect(events, isEmpty);
+    });
+  });
+
+  group('detectSystemMessages - failsafe', () {
+    test('"Failsafe mode activated" beim Wechsel in Emergency (navMode 15)', () {
+      final events = detectSystemMessages(
+        previous: _frame(navMode: 3),
+        current: _frame(navMode: 15),
+        now: now,
+      );
+      expect(events, hasLength(1));
+      expect(events.single.severity, SystemMessageSeverity.critical);
+      expect(events.single.message, 'Failsafe mode activated');
+    });
+
+    test('kein erneutes Ereignis, solange Emergency anhaelt', () {
+      final events = detectSystemMessages(
+        previous: _frame(navMode: 15),
+        current: _frame(navMode: 15),
+        now: now,
+      );
+      expect(events, isEmpty);
+    });
+  });
+}