fix: autocomplete popup repositions on window resize

[mberrishdev]

Fixes #4829

Problem

The autocomplete popup (shown when typing / or @) had a static position that didn't update when the terminal window was resized, causing it to appear misaligned.

Solution

Added reactive position tracking that:

• Polls the anchor element coordinates every 50ms when the popup is visible
• Only triggers re-renders when position actually changes (no performance impact when static)
• Uses useTerminalDimensions() to also track dimension changes
• Cleans up the interval when the popup closes