Separate Free extension boundary and refine preview UI

[charlessnow]

What changed

• adds generic public extension contracts and runtime hooks for official builds
• keeps workspace, licensing, and paid-feature implementations out of the Free build graph
• improves preview readiness, window behavior, renderer customization, and related UI
• adds standard macOS bundle metadata and regression coverage

Why

PeekMark uses an open-core model. The public repository needs stable, generic extension points while preserving a strict source boundary around proprietary Pro features.

Impact

The Free app remains independently buildable and retains its Markdown preview features. Official builds can attach private implementations at compile time without exposing Pro source in this repository.

Validation

• swift test — 10 tests passed
• unsigned Debug app build — succeeded
• public/private boundary check — passed
• staged secret/private marker scan — passed