Add vim keybindings mode

[rohitgs28]

Summary

Optional vim-style keybindings for power users. Controlled by config: vim_mode = true

Scope

• Normal/Insert mode toggle in query input
• j/k for scrolling results and schema panels
• / to start a search in results
• dd to clear query input
• Esc returns to normal mode
• Visual indicator in status bar (NORMAL / INSERT)

Technical Notes

• Add vim_mode: bool to AppConfig
• Mode state machine in app.rs
• Status bar already exists in renderer.rs

This is a high-impact feature. Vim users are the most vocal open source supporters and will star/share the project.