quality-debt: document 782px WordPress mobile admin breakpoint (#87)

* fix: document 782px WordPress mobile admin breakpoint in media query

Adds an inline comment explaining that 782px is the WordPress mobile
admin breakpoint (wp-admin responsive threshold), addressing the
maintainability feedback from Gemini in PR #68.

Closes #69

* fix: add empty line before CSS comment to satisfy comment-empty-line-before rule

CodeFactor flagged missing empty line before the breakpoint comment (comment-empty-line-before).
Also simplifies comment text per Gemini Code Assist suggestion (removes redundant parenthetical).

Fixes CodeFactor FAILURE and Codacy ACTION_REQUIRED on PR #87.

admin/css/admin-styles.css

@@ -124,6 +124,8 @@
 }
 
 /* Responsive Styles */
+
+/* 782px is the WordPress mobile admin breakpoint. */
 @media screen and (max-width: 782px) {
     .wpst-form-table th {
         width: 100%;