Story 4.3: Implement Core Navigation Controls (Next/Previous Slide)

[ajelinek]

Part of #10

As a user, I want simple "Next" and "Previous" controls to navigate linearly through the top-level slides of a presentation.

Acceptance Criteria:

• Visible "Next" and "Previous" buttons/UI elements are present in the presentation viewer.
• Clicking "Next" advances to the next slide in the sequence.
• Clicking "Previous" returns to the previous slide.
• Controls are disabled or hidden appropriately at the beginning/end of the presentation.
• Navigation updates the current view to show the correct slide.

Test Cases:

• E2E test: Clicking "Next" shows the next slide's content.
• E2E test: Clicking "Previous" shows the previous slide's content.
• E2E test: "Previous" is disabled on the first slide; "Next" is disabled on the last slide.

Constraints:

• Navigation state needs to be managed (client-side if interactive, or via URL changes).

Dependencies:

• Story 4.1 (Presentation Viewer Page), Story 4.2 (Render Individual Slides).