Enable/disable clips

Background:

• Clips (and other Items) have an enabled flag which toggles whether they are enabled or disabled. For audio clips, this is a mute/unmute toggle. For video clips, a hide/show toggle.

Problem:

• Currently Raven's display shows no indication whether a clip is enabled or disabled.

Feature Request:

• Raven should draw disabled Clips in a muted grey color so the enabled/disabled state is clear at a glance.
• In the Inspector tab, Raven should show an "Enabled" checkbox which can be toggled on/off to modify the currently selected Clip (or other Item).

Stretch goal:

• Tracks are also Items, and can be disabled. Raven should draw whole Tracks with a ghosted/grey style.

Estimated Difficulty:

• Easy
• Some familiarity with C/C++
• Only 2 source files need to be modified (timeline.cpp and inspector.cpp)
• Data structure is already in place.
• Test data is available.
• Only user interface needs to be changed.
• No difficult design or architecture choices.
• This task is a good introduction to Dear ImGui

Start with timeline.cpp and inspector.cpp, then inspect the existing enabled flag and available test data. Update the UI so disabled Clips or Items are visibly muted and the Inspector includes a toggle for the selected Item; if feasible, apply the same ghosted style to disabled Tracks.