Restore Sketcher line style dropdown release note

[newmattock]

Summary:
Restores the FreeCAD 1.2 release-note entry for FreeCAD/FreeCAD#30151.

Related bounty or issue:
Refs #94.

What changed:

• Documented that Sketcher line-style dropdowns keep their entries visible when the active theme changes, including themes without a Qt style sheet.
• Kept the update limited to the root Release_notes_1.2 page.

Validation:

• git diff --check origin/main..HEAD -- wiki/Release_notes_1.2.wikitext
• Confirmed current origin/main does not contain Sketcher: fix line style dropdowns being empty FreeCAD/FreeCAD#30151.
• Confirmed the branch diff against current origin/main is one release-note insertion.

Notes:

• An older merged PR (Document Sketcher line style dropdown fix #133) added this source PR previously, but the entry is absent from current main. This PR restores the missing current-main release-note entry.
• Translation files were intentionally not modified.
• This is a focused release-note addition and does not claim the broader Sketcher documentation issue is complete.