refactor: replace dmc.Drawer with dmc.Modal for filter selection (Task 7.4 + 7.5)

- Created 3 separate modals: Drug Selection (lg), Trust Selection (sm),
  Directorate Browser (xl) with centered overlay
- Added filter trigger buttons to filter bar with count badges
- Added "Clear All" button in filter bar for global filter reset
- Per-modal clear buttons for drugs and trusts
- Preserved all existing selection logic (same component IDs)
- Deleted drawer.py component and callbacks (replaced by modals.py)
- Updated CSS: filter-btn styles, modal chip/badge styles

dash_app/callbacks/__init__.py

@@ -6,9 +6,9 @@ def register_callbacks(app):
     from dash_app.callbacks.filters import register_filter_callbacks
     from dash_app.callbacks.chart import register_chart_callbacks
     from dash_app.callbacks.kpi import register_kpi_callbacks
-    from dash_app.callbacks.drawer import register_drawer_callbacks
+    from dash_app.callbacks.modals import register_modal_callbacks
 
     register_filter_callbacks(app)
     register_chart_callbacks(app)
     register_kpi_callbacks(app)
-    register_drawer_callbacks(app)
+    register_modal_callbacks(app)