refactor(shell): decompose 1760-line MainScreen into dedicated coordinators #1503

Problem

lib/features/main_screen/main_screen.dart is 1,765 lines long. Its state class _MainScreenState is responsible for too many unrelated subsystems:

1. Global Keyboard Event Routing: Listening to HardwareKeyboard, intercepting shortcuts (Ctrl/Cmd+P, Ctrl/Cmd+K, sidebar toggle, query run, escape).
2. Layout & Split Management: Managing _MainContentSplitState, sidebar show/hide animations, divider drag delta, width persistence.
3. Dialog & Flow Coordination: Welcome tour dialog trigger, URL connection import dialog, connection creation modal flow, unsandboxed driver consent gate.
4. Platform & Window Integration: Safe mode auto-relock timers, bitsdojo window title bar, macOS platform menu bar callbacks, and status bar bindings.

This makes MainScreen fragile to changes, increases widget rebuild scope, and complicates regression testing.

Scope

Decompose MainScreen by extracting dedicated controllers/coordinators:

• MainScreenKeyboardRouter: Handles global keyboard shortcuts and dispatches actions to command hosts.
• MainScreenDialogCoordinator: Manages modal flow triggers (connection creation, URL dialog, welcome tour, safe mode lock).
• MainScreenLayoutCoordinator: Encapsulates sidebar visibility notifier, split drag delta, and width persistence.

Acceptance

• main_screen.dart acts purely as a structural layout coordinator under 400 lines.
• Keyboard shortcuts, dialog flows, and split resizing logic covered by isolated unit/widget tests.
• E2E shell tests (test/e2e/06_shell_theme_and_command_palette/) remain green.