Story 4.6: Implement Hierarchical Navigation (Drill-Down/Return to Parent)

[ajelinek]

Part of #10

As a user, when a slide has nested (child) slides, I want to be able to navigate into them (drill-down) and then navigate back to the parent slide.

Acceptance Criteria:

• If a current slide has child slides, UI indicators or controls are available to navigate "down" into the first child slide.
• When viewing a child slide, UI indicators or controls are available to navigate "up" to its parent slide.
• This navigation integrates with keyboard (Up/Down arrows) and potentially touch (Swipe Up/Down) controls.
• The presentation state correctly reflects the current position in the hierarchy.

Test Cases:

• E2E test: Navigate into a nested slide stack and back out to the parent.
• E2E test: Navigation state (e.g., URL fragment, breadcrumbs) updates correctly during hierarchical navigation.

Constraints:

• Requires a clear visual representation of hierarchy and navigation options.

Dependencies:

• Story 3.3 (Build Hierarchical Navigation Tree).
• Story 4.3 (Core Navigation Controls), Story 4.4 (Keyboard Navigation).