Label generator: back-side codes, pictures, two-column layout, measurement guards

CHANGELOG.md

@@ -51,6 +51,18 @@ ADR-007 and ADR-002.
 
 ### Fixed
 
+- **A zero bar height printed a barcode 176 wide and 0 high.** Nothing rejected
+  it: the value went straight into a CSS length, `height: 0in` is perfectly
+  valid, and an image at `height: 100%` of a zero-height box is zero pixels
+  tall. A cleared field was its own version of the same bug - `'' + 'in'` is not
+  a length, the browser dropped the declaration, and the box fell back to auto
+  height. Every measurement now goes through `labelVars()`, which will not emit
+  NaN, an empty length, or a zero where a zero means the box stops existing;
+  padding, quiet zone and gap still accept zero, because there it is a real
+  answer. The stylesheet carries its own fallbacks too, though those only cover
+  a property that is absent - a property set to garbage is dropped at computed
+  value time and takes the fallback with it, which is why the sanitiser is the
+  fix and the CSS is only a net.
 - **A caption could eat the barcode.** The code box and the caption both grew,
   so any caption took half the label and kept going: "BAY 12 PRESS LINE 1" on
   1.00 x 0.50in stock left a 36px stub, which is a code that is present,