Fix invalid font shorthand in Tables demo sample

.header { font: 12pt; } is invalid CSS - the font shorthand requires
font-family, so the whole declaration was silently dropped (confirmed:
the header row rendered at the inherited 10pt instead of 12pt), which
cascaded into row-height/position shifts across the rest of the page.
Changed to font-size: 12pt, preserving the original intent (just set
the text size) as valid CSS.

Integration test suite: 26/29 -> 27/29 passing (Tables now matches its
baseline exactly, no baseline update needed).

Author: jhaygood86

Source/Demo/Common/TestSamples/13.Tables.htm

@@ -3,7 +3,7 @@
     <head>
         <title>Tables</title>
         <style type="text/css">
-            .header { font: 12pt; }
+            .header { font-size: 12pt; }
 
             .key {
                 color: #404080;