feat: add search and replace in the code editor

[Jatin-Shihora]

There's no way to find or replace text in the editor. If you have a long solution and want to rename a variable or find where you used a function, you have to scroll through manually. This gets painful with longer solutions.

What should happen

• Ctrl+F opens a search bar at the top or bottom of the editor
• Typing in the search bar highlights all matches in the code and jumps to the first one
• Enter or arrow keys cycle through matches
• Ctrl+H opens search + replace mode with a second input field
• Esc closes the search bar

Things to consider

• The search bar needs its own input handling separate from the editor. When search is active, typed characters go to the search input, not the code buffer
• Highlight matches using reverse video or a distinct background color via blessed's formatting
• Match highlighting needs to work with Pygments syntax highlighting already applied to the code. You'll need to overlay match colors on top of syntax colors
• Case-insensitive search by default, with an option to toggle case sensitivity
• The editor currently renders in render() in tui/editor.py. Match highlights would be added during the token rendering loop