Add keyboard shortcuts for navigation

[AmitoVrito]

Description

Add keyboard shortcuts for common navigation.

What to do

• / or Ctrl+K — focus search
• ← / → — previous/next page
• t — toggle dark/light mode
• ? — show shortcuts help modal

[AmitoVrito]

Thanks for your interest! I will assign it to you. Before you start, could you give the repo a ⭐ on the main repo (https://github.com/SynapseKit/SynapseKit)? It really helps with visibility for a new project. Looking forward to your PR!