feat: add clear (×) button to target input (closes #197)

[Maqbool61]

What

Adds a small clear button inside the target input that appears when there is text and empties the field on click — handy on mobile where selecting-all-and-deleting is fiddly.

Changes (frontend/src/components/Sidebar.tsx)

• Added X to lucide-react imports
• Wrapped the target <input> in a relative div
• Added pr-7 padding to the input when target has text so the clear button never overlaps the typed content
• Clear button appears conditionally ({target && ...}), positioned absolute right-2 top-1/2 -translate-y-1/2
• Clicking calls setTarget(''), aria-label="Clear target" for accessibility

Verified

npx tsc --noEmit → 0 errors ✅

How to test

1. Type anything into the target field → × button appears on the right
2. Click it → field clears instantly
3. Empty field → no button shown
4. During a running scan → input is disabled, button correctly not shown (target is already set)

Closes #197