[ZEPPELIN-6384] Fix dropdown menu item links not clickable in full area

### What is this PR for?

#### Summary

- Fixed dropdown menu items in header where only text was clickable instead of the full item area
- Changed routerLinkActive to nzMatchRouter for proper NG-ZORRO integration
- Added CSS pseudo-element to expand clickable area of anchor tags

#### Cause

- Likely due to NG-ZORRO dropdown menu item anchor tag style changes that reduced the clickable area to just the text content.

#### Changes

- Updated `header.component.html`: Replace `routerLinkActive` with `nzMatchRouter` directive
- Updated `global.less`: Add `::before` pseudo-element to `.ant-dropdown-menu-item > a` to expand clickable area using absolute positioning

### What type of PR is it?
Bug Fix

### What is the Jira issue?
https://issues.apache.org/jira/browse/ZEPPELIN-6384

### Questions:
* Does the license files need to update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No


Closes #5124 from tbonelee/fix-dropdown-link.

Signed-off-by: ChanHo Lee <chanholee@apache.org>