disable: comment out em/rem unit options, keep only px and mm for print

Margins: px + mm only. Font sizes: px only.
em/rem buttons are commented out, not deleted, for potential future use.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/ElementPopup.vue

@@ -60,7 +60,7 @@
                 >
                   px
                 </button>
-                <button
+                <!-- <button
                   type="button"
                   :class="{ active: fontSize.unit === 'em' }"
                   :disabled="inheritanceLocked"
@@ -75,7 +75,7 @@
                   @click="updateFontSizeUnit('rem')"
                 >
                   rem
-                </button>
+                </button> -->
               </div>
             </div>
           </div>