feat: top menu bar for keybinding discoverability

[mpryor]

Summary

Add a menu bar at the top of the screen that exposes available actions and their keybindings, improving discoverability for new users who don't know the keybindings yet.

Problem

Users currently need to press ? to discover available keybindings. A persistent (but toggleable) menu bar would make actions visible at a glance, similar to how htop or mc show function key shortcuts at the bottom, or how nano shows common commands.

Features

• Top menu bar showing categorized actions with their keybindings (e.g. Delimiter, filter, sort, /search, Parse)
• Menu items should be clickable (ties into feat: mouse support #38 mouse support)
• Hidden by default — toggled via user configuration in config.json (e.g. "show_menu": true)
• Menu should respect the active keymap (show correct bindings for vim/less/emacs/custom)
• Menu should respect the active theme colors

Technical Considerations

• Use Textual's Header or a custom Static widget docked to the top
• Categories could mirror the help screen sections: Navigation, Search & Filter, Columns & Data, Buffers, Output
• Consider a compact single-line format vs. expandable dropdown menus
• Related: feat: mouse support #38 (mouse support)