Add which-key.nvim for keybinding discovery

[ladislas]

Summary

Add which-key.nvim as a nice-to-have plugin to help discover and remember keybindings.

Motivation

This was listed as a nice-to-have in #1 but not implemented. With many leader-based and function-key bindings, a popup showing available keys would improve discoverability — especially for less frequently used mappings.

Details

• Add folke/which-key.nvim to a new or existing plugin spec file (e.g., lua/plugins/ui.lua)
• Configure it to trigger on leader key press with a short delay
• No custom bindings needed beyond defaults; the plugin reads keymaps automatically

Acceptance Criteria

• which-key.nvim installed and configured via lazy.nvim
• Pressing <leader> shows available keybindings after a short delay
• Existing keymaps are displayed correctly with descriptions