Rebuild TUI as 4-tab layout with 10 new widgets

Replace sidebar + 5-screen layout with horizontal tab bar (F1-F4)
and persistent StatusStrip. Consolidate Position + Scan screens into
Signal (F3) with Monitor/Sweep/Sky Map sub-modes via ModeBar.

Screens:
  - F1 Dashboard: system health, tracking panel, quick actions, presets
  - F2 Control: motor tuning, compass rose, preset management
  - F3 Signal: RSSI monitor, 1D sweep (SweepPlot), 2D sky map (heatmap)
  - F4 System: NVS editor with regex filter, EEPROM, firmware info
  - F5 Console: push/pop overlay (no longer a tab)

New widgets: StatusStrip, ModeBar, SweepPlot, QuickActions, PresetList,
ReceiverInfo, MotorTuning, NvsFilter, SystemHealth, TrackingPanel.

Removed: PositionScreen, ScanScreen, DeviceStatusBar (functionality
absorbed into new screens and StatusStrip).

App-level position poll feeds StatusStrip and active screen at ~2 Hz.
Fix shared threading.Event across instances (class-level mutable default).

tui/tests/test_sweep_stop.py

@@ -134,8 +134,7 @@ async def test_sweep_restart_after_stop():
 
         assert not signal._sweeping, "Sweep still running after polling"
         assert len(signal._sweep_data) > 0, (
-            f"First sweep should have data; "
-            f"device type={type(signal._device).__name__}"
+            f"First sweep should have data; device type={type(signal._device).__name__}"
         )
 
         # Second sweep.