fix: improve Labels filter UX (#14512)

[sanjay78632]

✨ Description

This PR improves the UX of the Labels filter component to provide clearer feedback when no labels are selected, making it consistent with other filters like the Namespace filter.

Changes made:

• Always display "Active Key/Value Pairs" section (previously hidden when empty)
• Show "none" text when no labels are selected
• Always show "Add" button (previously only visible after typing)
• Changed button text from "Add key/value pair" to "Add" for brevity
• Added counter at bottom showing number of selected labels (e.g., "0 Labels", "1 Label")
• Fixed capitalization consistency to use "Key/Value" throughout the component
• Added disabled state to "Add" button when inputs are empty for better UX

🔗 Related Issue

Closes #14512

🎨 Frontend Checklist

• [] Code builds without errors (npm run build)
• All existing E2E tests pass (npm run test:e2e)
• Screenshots or video recordings attached showing the UI changes

🛠️ Backend Checklist

This PR does not include any backend changes.

📝 Additional Notes

Before:

• Labels filter showed empty inputs with no indication of active filters
• "Add" button only appeared after typing
• No counter showing how many labels were selected
• Inconsistent capitalization (key/value vs Key/Value)

After:

• "Active Key/Value Pairs" section always visible
• Shows "none" when empty (matching Namespace filter behavior)
• "Add" button always visible (disabled when inputs empty)
• Counter at bottom: "0 Labels" → "1 Label" → "2 Labels"
• Consistent "Key/Value" capitalization throughout

Testing:
Tested locally at http://localhost:5173 - the filter now clearly indicates when no labels are selected and provides better visual feedback throughout the label selection process.