Implements the MainScreen class with:
- Dracula theme support (THEME = "dracula")
- 3-state sidebar (hidden/visible/fullscreen) with state transitions
- Custom layout components: Throbber, SessionTabs, FlashBar, Sidebar
- No generic Header/Footer widgets
- Responsive layout with proper CSS styling
- Full type annotations for pyright compliance

Includes comprehensive BDD tests in Behave format covering:
- Theme initialization and verification
- Sidebar state transitions and display
- Session tab management (add, remove, switch)
- Flash bar message display with different types
- Throbber loading indicator rendering
- Layout responsiveness to sidebar state changes

@@ -0,0 +40,4 @@
+    """Mount the MainScreen."""
+    context.screen = MainScreen()
+    # Simulate mount
+    context.screen.post_message(context.screen.on_mount())

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+    """Verify the screen has the specified widget."""
+    widget_map = {
+        "session tabs": SessionTabs,
+        "sidebar": Sidebar,

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+Feature: TUI MainScreen with Dracula theme and 3-state sidebar

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+"""MainScreen implementation with Dracula theme, 3-state sidebar, and custom layout components."""
+
+from enum import Enum
+from typing import Optional

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+        self._spinner_frames = ["⠋", "⠙", "⠹", "⠸", "⠼", "⠴", "⠦", "⠧", "⠇", "⠏"]
+        self._current_frame = 0
+
+    def render(self) -> str:

@@ -0,0 +307,4 @@
+        background: $surface;
+        color: $text;
+        border: solid $accent;
+    }