Implement CSS box-sizing for HTML layout sizing

Add CSS box-sizing support for HTML widgets and route CSS-specified
width/height conversion through virtual sizing hooks. Resolve percentage
CSS sizes against the containing block content box, and apply content-box
or border-box conversion consistently across block, flex, grid, table,
and rich text layout paths.

Keep table wrapper width behavior compatible with CSS table layout by
overriding the table width conversion separately, so padded percentage
tables do not overflow their assigned wrapper width.

Add focused regressions for block padding, flex-basis sizing, grid
containers, table containers, and the border rendering cases that depend
on explicit border-box sizing.

.ecode/project_build.json

@@ -377,7 +377,7 @@
         "working_dir": "${project_root}/bin"
       },
       {
-        "args": "-c system --hn-dark",
+        "args": "--hn-dark",
         "command": "${project_root}/bin/eepp-ui-html-debug",
         "name": "eepp-ui-html-debug",
         "working_dir": "${project_root}/bin"