Add unit coverage for sidebar layout and session behavior

[KSHITIZ6341]

Summary

Current tests cover SettingsStore and PinSessionStateMachine, but not the higher-risk sidebar session behavior.

Current behavior

There are only two test files:

• Tests/SwitcherTests/SettingsStoreTests.swift
• Tests/SwitcherTests/PinSessionStateMachineTests.swift

High-risk behavior in PinSessionManager is currently untested.

Suggested coverage

Add tests for:

• width clamping per display
• stack layout for 1, 2, and 3 pinned apps
• unpin/reorder behavior
• moved/resized manual break detection
• persistence of edge/display/width settings
• closed-window removal behavior where feasible

Suggested implementation approach

• Extract pure layout helpers where useful.
• Introduce injectable display/window doubles for session tests.
• Keep UI/AppKit integration tests manual for now.

Acceptance criteria

• Sidebar layout/session logic has focused unit tests.
• Tests avoid requiring live third-party app windows.
• CI can run the test suite reliably.