Add DemoCraftClient for complete offline demo mode

- DemoCraftClient in demo.py: duck-typed CraftClient replacement with
  8 canned satellites, synthetic pass predictions, and time-varying LEO
  arcs that drive real AOS/TCA/LOS pass events to the camera overlay
- Fix sky map EL signal fidelity: snap _el to _target_el at sweep start
  so 2D scans compute correct per-row RSSI (was using stale elevation)
- Branch on demo_mode in app.py _setup_craft_client() to inject
  DemoCraftClient instead of the HTTP CraftClient
- Add test_demo_craft.py: 5 tests exercising search, passes, tracking,
  and WAITING state through the full TUI without mocks
- Update take_screenshots.py to cover all 8 screens (dashboard, control,
  craft search, craft tracking, signal, system, console, camera)

tui/src/birdcage_tui/app.py

@@ -141,9 +141,14 @@ class BirdcageApp(App):
 
     def _setup_craft_client(self) -> None:
         """Create a Craft API client and hand it to the control screen."""
-        from birdcage_tui.craft_client import CraftClient
+        if self.demo_mode:
+            from birdcage_tui.demo import DemoCraftClient
 
-        client = CraftClient(base_url=self.craft_url)
+            client = DemoCraftClient()
+        else:
+            from birdcage_tui.craft_client import CraftClient
+
+            client = CraftClient(base_url=self.craft_url)
         try:
             control = self.query_one("#control")
             if hasattr(control, "set_craft_client"):