refactor(app): move imports to module level for better testability

Move internal imports in run_pipeline_mode() to module level to support
proper mocking in integration tests. This enables more effective testing
of the app's initialization and pipeline setup.

Also simplifies the test suite to focus on key integration points.

Changes:
- Moved effects_plugins, DisplayRegistry, PerformanceMonitor, fetch functions,
  and pipeline adapters to module-level imports
- Removed duplicate imports from run_pipeline_mode()
- Simplified test_app.py to focus on core functionality

All manual tests still pass (border-test preset works correctly).

engine/app.py

@@ -5,13 +5,24 @@ Application orchestrator — pipeline mode entry point.
 import sys
 import time
 
+import effects_plugins
 from engine import config
+from engine.display import DisplayRegistry
+from engine.effects import PerformanceMonitor, get_registry, set_monitor
+from engine.fetch import fetch_all, fetch_poetry, load_cache
 from engine.pipeline import (
     Pipeline,
     PipelineConfig,
     get_preset,
     list_presets,
 )
+from engine.pipeline.adapters import (
+    RenderStage,
+    SourceItemsToBufferStage,
+    create_items_stage,
+    create_stage_from_display,
+    create_stage_from_effect,
+)
 
 
 def main():
@@ -45,18 +56,6 @@ def main():
 
 def run_pipeline_mode(preset_name: str = "demo"):
     """Run using the new unified pipeline architecture."""
-    import effects_plugins
-    from engine.display import DisplayRegistry
-    from engine.effects import PerformanceMonitor, get_registry, set_monitor
-    from engine.fetch import fetch_all, fetch_poetry, load_cache
-    from engine.pipeline.adapters import (
-        RenderStage,
-        SourceItemsToBufferStage,
-        create_items_stage,
-        create_stage_from_display,
-        create_stage_from_effect,
-    )
-
     print("  \033[1;38;5;46mPIPELINE MODE\033[0m")
     print("  \033[38;5;245mUsing unified pipeline architecture\033[0m")